I read that the GH:WT guitars will be compatible with RB2. Not sure about vice-versa though.
from what I read, you got that backwards.
I don't know about PS3, but it has been announced for 360 by MS that all the music peripherals will be compatible with either game.
Dunno if that is MS layin down the law on their system or these companies just realizing how ridiculous it is to expect people to by 20 sets of "instruments".
[EDIT]
I just read that the MS announcement was an "us too" announcement in response to one from Sony the day before saying the same thing, so it looks like the future of rhythm game instrumentation is bright on both platforms.