Could be true, and if it is, Capcom are taking another huge risk, even bigger than the one they took when making RE4 a GC exclusive - before they changed their minds.
Unless MS has made some transactions or quarantees to that direction. Also Dead Rising's good performance reduces the risk somewhat. Still I'd wait for something more concrete information to appear before I'll let the horses go.
Looks like this was another case of "we won't say it's exclusive or not to make peaple think it's exclusive." Lucky they didn't say it was like the case with FIFA and Eleven soccer games.