I was informed today of the existance of RB2. As much as I love Harmonix, it looks like they're taking the Neversoft approach, which consists of taking features from the competing game and making them bigger (the only exception being the lack of create-a-song mode). This puts the average buyer (i.e., one without millions of dollars to spend on music games and peripherals) in quite a tough spot. Do you buy Rock Band 1 or 2? Or do you buy GH:WT? Each one is likely to cost exorbitant amounts of money, and the peripheral sets are quite different (especially the drums), so you can't simply buy four peripherals and a copy of each game (well, the RB instruments will be compatible with each other, at least).
But there's the issue of: is a sequel really necessary? I consider Rock Band a perfect game in that it sets out to do a certain thing and succeeds in every field imaginable, so why not just keep releasing songs? The only way I can find this acceptable is if Harmonix makes the downloadable songs compatible with both games (it would suck to be forced to play one song on one game only), and I'm not sure that's feasible.
I'm sure info has changed by now, so feel free to correct me on whatever.