North American and European Release Dates
There's no exact date set in place yet. We only have the Spring 2010 timeframe to go on. Could be around March or April.

THE POKEWALKER
The Pokewalker is a pedometer where you transfer one Pokemon to the device to increase its EXP and raise its happiness level. There are two features you can take advantage of while using the Pokewalker: Itemfinder, where you can have the chance to find an item, and capturing Pokemon, which is a barebones battle and capture. Of course, you can't do those freely. As you walk (yeah right), you gather Watts, the Pokewalker's currency. This enables you to participate in battles or finding items. 3 Watts for a shot at an item, 10 Watts to battle a Pokemon.

Don't doubt the limitation of the Pokewalker. It actually has a wide variety of courses and items for you to get, ranging from Hoenn and Sinnoh Pokemon to TMs, hold items, and so on. A full adventurer will hold 3 captured Pokemon and 10 items on store until they're transferred back to the Heart Gold/Soul Silver cart.

You can choose which course your Pokemon will go through (additional courses are purchased for several thousand Watts). There are also special courses but these two are the only ones readily known:
Edge of the Night's Sky: Unlocked by transferring an event Jirachi.
Yellow Forest: Unlocked with a Wi-Fi event. The rarest find in this area is finding a Pikachu that knows Volt Tackle, Surf, and Fly.

STARTER POKEMON
In addition to the Johto starters Cyndaquil, Chikorita, and Totodile, you now can get one of the Kanto starters (Charmander, Bulbasaur, Squirtle) and one of the Hoenn starters (Treecko, Mudkip, Torchic). The safe way to make sure you can get a shot at getting one is to defeat Red. Once that's done, see Professor Oak for a Kanto starter and Steven Stone in Silph Co. at Saffron City for a Hoenn starter.

LEGENDARIES
Lugia - Whirl Islands: HG lvl 70/SS lvl 45
Ho-oh - Tin Tower: HG lvl 45/SS lvl 70
Entei - Roams Johto after you enter the Brass Tower @ lvl 40
Raikou - Roams Johto after you enter the Brass Tower @ lvl 40
Suicune - Follow it from north of Cianwood, then the entrance of Mt. Mortar, then Vermillion, then Route 14, and finally encounter it east of Bill's house on Route 25 after Misty returns to her gym. Level 40.
Articuno - Seafoam Islands with 16 badges. Level 50.
Zapdos - Power Plant with 16 badges. Level 50.
Moltres - Silver Cave with 16 badges. Level 50.
Groudon - After receiving the National Pokedex and a Kanto starter, receive an orb from Mr. Pokemon. Then traverse a cave at Route 47. Level 50 in Soul Silver only.
Kyogre - After receiving the National Pokedex and a Kanto starter, receive an orb from Mr. Pokemon. Then traverse a cave at Route 47. Level 50 in Heart Gold only.
Rayquaza - You need to have both Kyogre and Groudon in your party and both must have been captured in their respective DS game, not from Ruby, Sapphire, or Emerald. Show the two to Professor Oak to receive another orb. Return to where you fought Kyogre or Groudon and Rayquaza will be there. Level 50.
Dialga, Palkia, or Giratina - Take an event Arceus to the Ruins of Alph (well, somewhere near it) where you partake in a ritual to make one of the three. They also know one unique move that they don't learn by level up or TM. Given to you at Level 1. NOTE: This is the only way to get a Griseous Orb in Heart Gold and Soul Silver as some sort of brain fart at Game Freak is preventing the item to be untradeable even when the game contains the freaking item. Therefore, if you want to bring in a Giratina in its Origin Forme in Heart Gold or Soul Silver, you NEED to pick Giratina for the ritual.
Latios - After receiving the National Pokedex, enter and exit the Pokemon Fan Club in Vermillion City where Steven Stone will alert you that Latios is roaming Kanto. Alternatively, an item in the game can summon it to a battle on the spot but it is not known how to get the item yet. Level 35.
Latias - After receiving the National Pokedex, enter and exit the Pokemon Fan Club in Vermillion City where Steven Stone will alert you that Latias is roaming Kanto. Alternatively, an item in the game can summon it to a battle on the spot but it is not known how to get the item yet. Level 35.
Mewtwo - After defeating Red, you will find it at the usual place: Cerulean Cave. Level 70.

VERSION EXCLUSIVES
Heart Gold: Mankey, Primeape, Growlithe, Arcanine, Spinarak, Ariados, Gligar, Gliscor, Mantine, Mantyke, Phanpy, Donphan, Sableye, Baltoy, Claydol
Soul Silver: Vulpix, Ninetales, Meowth, Persian, Ledyba, Ledian, Teddiursa, Ursaring, Delibird, Skarmory, Mawile, Gulpin, Swalot

In essence, HG gets shafted. =\

REGARDING ALTERNATE FORMS
Rotom - Have it lead your team before you enter the elevator in Silph Co.
Giratina - Lack of forethought or common sense at Game Freak prevents the Griseous Orb to be traded entirely. You need to get a Giratina from the Arceus event to get a new Griseous Orb.
Shaymin - Get the Gracidea Flower from the Goldenrod Flower Shop. Sky Form is active from 5am to 8pm.
Deoxys - The meteorites that change Deoxys are in the southwest portion of Route 3:
-Upper Left: Normal
-Upper Right: Attack
-Lower Left: Defense
-Lower Right: Speed

GYM LEADER REMATCHES
All 16 gym leaders found in Johto and Kanto are present and accounted for. Here's how to find them for a rematch:
Falkner - Daytime on Mondays, find him in the Celadon Department Store 4F with Janine. Rematches on Saturday mornings.
Bugsy - Daytime on Thursdays, find him near the south entrance of Viridian Forest. Rematches on Thursday afternoons.
Whitney - Daytime on Saturdays, find her in the Goldenrod Department Store 6F. Rematches on Saturday afternoons
Morty - Daytime on Mondays, find him next to the Tin Tower. Rematches on Tuesday nights.
Chuck - Talk to his wife outside the gym. Rematches on Wednesday nights.
Jasmine - Out everyday in the afternoon. Find her in the Olivine diner. Rematches on Wednesday afternoons.
Pryce - Out at the Lake of Rage on Saturday mornings. Rematches on Monday morning.
Clair - Out during most mornings in the Dragon's Den by the water. Rematches on Friday nights.
Brock - Out during most afternoons in Diglett's Cave at the Route 2 entrance. Rematches on Saturday nights.
Misty - Out during most evenings at Route 25 by Bill's house (practically the same area where you fight Suicune). Rematches on Wednesday mornings.
Lt. Surge - Outside the Power Plant from 9am to 11am every day. You need to show him a Pikachu. Rematches on Friday mornings.
Erika - Out on Sunday afternoons at Celadon City's fountain with Jasmine. Rematches on Sunday mornings.
Janine - Friday nights at the reception gate for Victory Road. Rematches on Monday afternoons.
Sabrina - Friday nights at the Olivine City Harbor. Rematches on Sunday afternoons.
Blaine - The fiery dapper man is out Tuesday mornings at Cinnabar Island. Rematches on Tuesday afternoons.
Green - Show a Pokemon with maxed out happiness to his sister, Daisy Oak, in Pallet Town. Rematches on Sunday nights. Green does leave the gym on Fridays for Cinnabar.

Your rematches will take place in the Fighting Dojo in Saffron City.

Rematches with gym leaders have a full team of Pokemon whose levels scatter around between 50 and 60 with a few outliers here and there. Green is the exception with his Pokemon mostly near Level 70.
Almost every gym leader has at least 1 Pokemon from every generation.

ELITE FOUR REMATCHES
Simply defeat Red. Every member will have a full and updated roster.

RED
His Espeon has been mysteriously switched out for an inferior replacement: Lapras. Also, Hail will now be in effect for the fight, affecting every member of his team but the Lapras.

BATTLE FRONTIER
For some tremendous and lazy reason, Game Freak opted to just copy and paste Pokemon Platinum's Battle Frontier. It is the EXACT same one: same battle styles, Frontier Brains, tutors, prizes...everything.

POKETHLON
Bored? Have some minigames! (Yay...) Well, at least you have a shot at some of the rarer items to get in Pokemon games including Heart Scales, Metal Coats, Nuggets, and even Rare Candies. Everyday though you get a shot at some evolution-inducing stones or a colored apricorn of your choice.

Boost your Pokemon's stats for the games as if you were raising them for a Super Contest so they'll perform better. (And you'll probably maybe want to cheat by using a second stylus with your other hand during play.)

SAFARI ZONE
The Safari Zone we've all been familiar with has now been given a refreshing update: now you can set up your preferred areas to visit at the time by getting to them much sooner so you don't end up wasting steps. And not only can you can unlock a whole new set of areas with new Pokemon in there as well, you can place items to lure out rarer Hoenn and Sinnoh Pokemon as well.

BERRY PLANTER
Since there isn't any soil around Johto or Kanto, you're in luck. Instead of repeating what happened in Fire Red and Leaf Green, you now have the new Berry Planter key item. Now you get to take your berries with you so it's a quick affair. Of course, the big downside here is that you only have 4 berry plants to raise at a time instead of (if you're crazy) all of the soil spots in a region.

APRICORNS
Making a return from Gold and Silver are the Apricorn Balls, where special Pokeballs are made from colored Apricorns. Kurt in Azalea Town will create one ball per day at no charge.
White - Fast Balls. If the wild Pokemon has more speed than your current one, you are 4 times more likely to catch it
Green - Friend Balls. The wild Pokemon caught will have a higher base happiness.
Black - Heavy Balls. Look up your Pokedex! This only affects Pokemon that weigh more than 440 pounds.
Red - Level Balls. The higher your level of your Pokemon against the wild Pokemon, the bigger the multiplier is.
Blue - Lure Balls. If you use it on a Pokemon that was lured in with a fishing rod, you are 3 times more likely to catch it.
Yellow - Moon Balls. You are 4 times more likely to catch a Pokemon that evolves with a Moon Stone.
Pink - Love Balls. Quite possibly the best ball of all, you are 8 times more likely to catch a Pokemon if they're the opposite gender of your current Pokemon.

There are also ApriShakes where you crush the apricorns to make a drink for your Pokemon. These affect its stats for Pokethlons.

You can't plant Apricorns but the trees that you find them off of will replenish their supply.

BUG CATCHING
Still there on Tuesdays, Thursdays, and Saturdays. Catch the best Bug-type Pokemon you can. More Pokemon added after you beat the Elite 4.

MOVE TUTORS
Pokemon Platinum featured three move tutors in three different areas of Sinnoh where you required to trade in colored shards for a certain move you wanted. Now three different tutors who teach the same series of moves are housed in a single area in front of the Battle Frontier but now it's much worse: they must be paid with Battle Points earned from the Battle Frontier AND THE PRICES ARE HIGH. Prices are locked in at 32, 40, 48, and an incredibly unreasonable 64 BP! Why such a heavy price, Game Freak?!

Someone in the Ilex Forest will endlessly teach Headbutt for free to any Pokemon capable of doing so. It's meant to bash the trees so look for wild Pokemon.

In Blackthorn City, housed in the same place with the Move Relearner and Move Deleter is someone who teaches the "Elemental Hyper Beams" only to starter Pokemon (not just the Johto ones, all 12 starters). The cost is simply showing a starter with maxed out happiness.

PC BOX BACKGROUNDS
http://www.filb.de/games/tools/aikotoba

This message was edited by Reenee on Sep 24 2009.

EVENT RELATED ITEMS AND POKEMON

Currently the origin names of these Pokemon are for their Japanese releases. When the actual event related to the Pokemon outside of Japan is known, I'll update the post.

Pokemon
12th Movie Arceus: Transfer this over and take it with you to the Ruins of Alph where a series of events will spawn a Level 1 Dialga, Palkia, or Giratina of your choice. Each one has a unique move that is not learned by normal means and they will also have their respective orbs in possession. Note that this is the only way to get a Griseous Orb for Giratina for use in Heart Gold and Soul Silver.

Details:
Level 100
Holds Rowap Berry
Judgment, Roar of Time, Special Rend, Shadow Force (the signature moves of itself, Dialga, Palkia, and Giratina)

Nintendo Zone Jirachi: Transfer this Jirachi to the game and you unlock the Edge of the Night's Sky course for your Pokewalker.

Details:
Level 5
Holds Liechi Berry
Wish, Confusion, Rest, and Draco Meteor (a move exclusive to Dragon-type Pokemon)

Pikachu-colored Pichu (aka Shokotan Pichu): A deliberately shiny Pichu that's used to unlock one of the most useless Pokemon next to Dunsparce: an untradeable, impossible-to-evolve Spiky-eared Pichu. (It was previously known as the Notched-eared Pichu as it was named in Japan.) But who cares about that when you've got a shiny Pichu that you can actually use?

Details:
Level 30
Jolly Nature
Holds Everstone
Charge, Volt Tackle, Endure, Endeavor

Celebi: Serebii.net, in all their vast wisdom, decided to spoil a precious bit of knowledge to flesh out one of the most revealing plot points regarding Team Rocket. One of their cohorts managed to create a Celebi with the needed triggers to partake in this event. That's all we know so far.

Details:
None yet. We're not even supposed to know this event exists in the first place!

--
OUTSIDE OF HEART GOLD/SOUL SILVER

We're behind Japan concerning their events. Or some other Pokemon game has a Pokemon for use in the main games. Either way, you most likely own the means to get at these event items or Pokemon.

PROF. OAK'S LETTER - POKEMON PLATINUM: From September 28th through November 8th, you can download this Key Item over Mystery Gift Wi-Fi to encounter Shaymin in the wild. You need to have access to Route 224 so you're going to have to defeat the Elite Four for this one.

This message was edited by Reenee on Sep 23 2009.

Annnnnnd we're good to go. Still going to add to the first post when I've got some free time (for example, linking to the moveset changes).

Feel free to ask questions, add details, or suggestions.

Notch Eared Pichu is apparently the Spiky Eared Pichu in NA.
Its still useless, but I wanted to share this lol.

I agree. Notched Ear sounds better. Also, who honestly uses shineys anyway? I just collect the few I can. I've got Shiney Pokemon on my Ranch. They look cool.

Can you trade pokemon from diamond or pearl to this game?

quote Funnyguy194

Can you trade pokemon from diamond or pearl to this game?

Yes.

So then technically you can get all pokemon in HG and SS, right?

quote Funnyguy194

So then technically you can get all pokemon in HG and SS, right?

Nope, since most of the pokemon from hoenn and sinnoh won't be in the game IE sinnoh starters/trapich...etc

So what would happen if you tried to trade them over?

Then you have them. He means that you can't catch them. So, yes, they are in the game if you trade them over.

Technically you can obtain all pokemon in the game through trading, right?

Yes. You could do that with Pal Park, aslong as you had Pokemon Coloseum, to get some of the pokemon.