Activision made kind of a big deal at E3 about Guitar Hero/Rock Band cross-compatibility. Said that, yes, you'd be able to use Rock Band instruments in Guitar Hero World Tour. But don't for a second think they're happy about it. Check out the fine print on the back of the Guitar Hero: World Tour box.
Activision authorizes use of this Guitar Hero Game only in combination with Guitar Hero controllers provided by Activision or standard gamepad controllers provided with the game console or other Activision licensed controllers. Any other use is expressly prohibited. No other license, express or implied, is granted.
Not just prohibited, expressly prohibited. Course, they can't actually prohibit you from doing squat, but just in case, if you're ever on the phone with Activision customer support, and they ask if you were using Rock Band instruments, you say "no". ‘Guitar Hero' And ‘Rock Band' Fine Print: Activision ‘Prohibit's Some Uses Of Guitars [MTV]