From Kotaku's interview with GHWT producer Brian Bright:
Wow...
Haven't we seen PR about bands being exclusive to one or the other?
Brian isnt afraid of having to win back any fans they might have lost to Guitar Hero IIIs bruising difficulty (he thinks Guitar Hero: Aerosmith has reassured those poor souls that Guitar Hero has come back from the Dark Side). Even if those players have gone over to Rock Band, theres room enough on everyones shelf for two games; especially with the instrument compatibility. Moreover, Brian doesnt know of any artists that have exclusive deals with Rock Band or with Guitar Hero, so beloved songs will always be available to both factions.
Wow...
Haven't we seen PR about bands being exclusive to one or the other?