Over on MTV's Multiplayer blog, they've collected some frequently asked Rock Band questions to get to the bottom of what would-be rockers want to know. After all, the MTV Multiplayer folks should have a certain special degree of access as Rock Band is being published by MTV Games; "coincidently [our Rock Band] source has the same bosses we do," admits the author of the post.
Unfortunately, their source wasn't entirely forthcoming. When asked if the Guitar Hero III Gibson Les Paul controller would work with Rock Band, they were told "Harmonix has an open platform philosophy and their games will be compatible with third-party controllers that conform to the various platform controller standards." Which doesn't really answer the question.