'13' days is what a new counter on diablo3.com and the Blizzplanet website says. That's May 1st, and looks like there is a very good chance that Diablo 3 will be officially announced. Underneath the counter it says "The Church of the Triune will be shaken in 13 days."

There are hardly any details or rumors about the game -- but many have thought that it is all but certainly being developed, over the last few years. Job postings from Blizzard websites over the last long while have recruited people to work on a projects  with "an experienced Game Designer to help lead the team that designed Diablo and Diablo II" and the "team behind Diablo I and II." And then there was a Diablo novelist, Richard Knaak, who in an interview dropped a hint that work was in progress:

Knaak: This is a pure collaboration between myself and Chris Metzen/Blizzard. All that is written is passed by him and the others there. This will be canon and has adjusted earlier info. The ramifications here will be used for any future project ... and I ain't writing for a dead game. :)

Question: So ... Diablo 3?

Knaak: I am not writing for a dead world ... but a world with dead. 'nuff said !  (source)

In typical Blizzard style, it would make sense not to announce the game until work was far underway. Some reports put the start of the Diablo project around 2002.

It could be a big game -- but it is not looking like it will be a MMO, due to certain tips dropped over the years. One pointer towards it being non-MMO is the possible size of the staff working on the game.  In an interview with Gamasutra, Blizzrard VP of Product Development was talking about the number of people working on various Blizzard projects in July of 2007: ""Our global headcount is 2,700," said Pearce, "And most of that is customer service for World of Warcraft! In terms of development staff it’s probably around 350. World of Warcraft is about 135 people, 40 for Starcraft II, 40 for team 3, our cinematics team is about 85 guys. Then there’s sound and Q/A and that sort of thing."

When pressed for details regarding for "team 3" he didn't reveal much more than to say that: "Team 3 is working on something really awesome. I can’t give you any hints, but it’s totally awesome."

Diablo 2 came out a while ago now -- 2001 -- but remains popular to play even today, and at the time, was one of the most popular online games going. Both PC Diablo games virtually spawned their own sub-genre of games, with many copies and imitations over the years of the loot-based, hub-centered, ARPG formula. Hellgate:London, Loki, the Marvel Super Heroes, Titan's Quest, Mythos, and many more have virtually copied the Diablo formula, and the game's success probably played a good part in Blizzard being able to raise enough capital to make World of Warcraft. In turn, Diablo was a late '90's update of some of the oldest computer dudgeon-crawlers known as Roguelike games, the most famous being Rogue (of course) and the still-being-developed, incredibly difficult Nethack game.