I have to admit that I've been pretty unhappy with Activision over the whole guitar incompatibility issue. Sure, I rock out on my 360 so I haven't had any issues, however, one of my best friends is more of a PS3 guy. He's only dropped the cash for a single Guitar Hero controller, however, his Rock Band Strat sits there begging to be used. Well it seems that this won't be such an issue for the next GH title.
Activision officially confirmed that Guitar Hero: World Tour will be compatible with “non-GH instruments”. When pressed, they went as far as to confirm that the instruments from the original Rock Band will indeed work. They haven't tested RB2 instruments (obviously), however, we know that RB1 and RB2 instruments will be interchangeable, so there shouldn't be any issues.
There's still no word on if this will affect previous versions of the game like GH III, however, we do know that a patch has been applied to GH: Aerosmith to allow RB controllers to work. Thumbs up to Activision for this one, because we only have room for so many plastic instruments.