Michael Shorrock, Sony's director of third-party relations, announced on the Playstation blog that he's been working with the major music game developers (i.e. Rock Band's Harmonix, Guitar Hero's Activision, and others) to ensure basic compatibility
for peripherals between the games. Joystiq has a compatibility matrix
that shows which devices work with which games.
"...Rock Band 2's guitar and drum set will work with Guitar Hero: World Tour and with Rock Revolution. Conversely, Rock Revolution's drum set will work with both Guitar Hero: World Tour and Rock Band 2. In addition, Guitar Hero: World Tour and Rock Band 2 will both support the SingStar microphones. We're still working hard to ensure compatibility between the Guitar Hero and Rock Band titles currently on the market, and we hope to have an announcement on that shortly."
Update: 8/20 17:38
notes that Microsoft has now made an official announcement
to do this as well.