http://kotaku.com/397427/rock-band-2-announced
So xBox 360 Rock Band comes out in September, PS3 is on its way, but no date.
Wii has no info, more then likely it'll end up the same as the first one D:
I own all of lastgen systems as of October 2008. (Finally got a Dreamcast)
I own every currentgen system except PS3.