I think they've tried to make everything compatible. I haven't heard of stuff not working, but some the the old GH games might not work, but that shouldn't be a problem if you're on a Wii. From a marketing standpoint, it's a good idea. No consumer wants to buy GH and RB controllers, so if you allow them to use the GH on RB and vice versa, the consumer will more likely buy both games.
Thanks to all for responding. I have a GHWT controller and I want to buy the new Beatles Rock Band game, but I would not if I had to buy a Rock Band guitar with it. I think the game will be really cool. Plus, it will expose a younger generation to some truly great music.
I'm thinking of getting the limited edition game with the guitar, drums, etc. I wish there were more reviews out there for the game specific to the Wii.
I got it yesterday and I'm loving it so far. I'm a pretty big beatles fan (mostly later stuff), so thats definitely a factor. I would agree with most of the reviews saying it seems easier than previous GH/RB games...I played expert on "I've got a feelin', and while it was hard I made it all the way through on the first try....which would not be the case on most GH:WT songs (for me at least).
But I don't play for the difficulty, I play because I love the songs.
BTW, I have the GH:WT guitar + drums and they work fine.