Ant Commandos has just announced the first guitar that can support both games on multiple platforms. Their Double Range Guitar plays PS2 and PS3 versions of both Guitar Hero (I, II and III) and Rock Band.
All the functionality is retained, like a second set of Rock Band solo buttons near the strum bar and distortion effects, plus you get some nice extras like wireless functionality and a fucking belt clip that lets you spin the guitar in 360s like ZZ TOP! I just wet myself! Available now for $59.99. Click pic for more details.