Sony announced today that they have been working closely with Neversoft and Activision, Harmonix and Konami to ensure that PS3 guitars and drum sets will all feature a basic level of gameplay compatibility.
That means you can use a Guitar Hero Guitar or Drum with Rock Band 2 software and vice versa.
The music hardware of Rock Band 2 and Guitar Hero are also compatible with Rock Revolution and vice versa.

Sony is still working to ensure compatibility between the Guitar Hero and Rock Band titles currently available.
This is a smart move by Sony to make music instruments compatible. This will cause more PS3 owners buy more music software. The money is in the software not in the plastic gear.
Via the offical Sony Playstation blog.