After bickering between Activision and Harmonix meant you couldn't use Guitar Hero's guitars with Rock Band on the PS3 ('cause Activision wanted moolah for Harmonix to release the patch), Sony has grabbed them both firmly by the nuts and said "everybody's instruments must work with everybody's games."
This means that Guitar Hero: World Tour's insane kit—guitars and drums—will work with Rock Band 2's and vice versa (and Konami's Rock Revolution series). Some of the advanced game-specific features might not work (like maybe the touch features on GH:WT's guitar), but basic compatibility will be there. They're working on making stuff already out there (GH3 and RB) play nice too. All I have to say is you rock, Sony. [PlayStation Blog via Kotaku]