While there's been no official announcement on the matter, Wired's Game|Life has been collecting evidence that Guitar Hero-killer Rock Band might be an Xbox 360 exclusive:
Clue #1 – The PlayStation logo is mysteriously absent from the bottom of the invitations to Rock Band's E3 party. The Xbox 360 logo is front and just-off-center, top row.
Clue #2 – Sony didn't include Rock Band in a recent press release that featured a list of PS3 games due by the end of the fiscal year. What, is Rock Band just not as big a deal as Dynasty Warriors: Gundam?
Of course, none of this evidence is conclusive. If Rock Band will be an exclusive title, we'll hear about it soon enough — E3 kicks off this week.