If she isn't breaking the skin, the risk is minimal. What should be considered, however, is the risk to her. If she has any kind of lesion in her mouth or on her lips, anything from a small cut to an open cavity in a tooth, bacteria on your arm could gain access to her bloodstream.

It's not a good idea, simply because there are so many ways in which bacteria could be introduced to either of you without either of you even knowing you were at risk at the moment.

Not from a medical point of view, purely empirical, but when I hooked up with a chick that was into biting, scratching, rough stuff, she turned out to be completely crazy. Won't post the details of what she did here or her emotional state, but let's just say, it wasn't normal. I don't know how long you've been dating your girlfriend, and this isn't a dating advice thread, but just keep an eye out for any other signs that something is a bit 'off'.