I could see them holding Crackdown back and releasing it alongside Scorpio--we know Crackdown was going to work sans cloud computing, so those cloud-computing features may be Scorpio-only features. If you don't have Scorpio, then you get the 'offline' version of the game that was going to be available sans cloud computing originally.
As for the original point of this thread...I thought there was discussion of some of the old Rare IPs being shopped around (e.g. Battletoads) for XBox exclusives by Microsoft.
Frankly, there are a lot of IPs that Microsoft has in acquiring Rare that they've just been sitting on that they could do *something* with (e.g. Jet Force Gemini, Blast Corps, Kameo, Viva Pinata, Perfect Dark)--it would be wise for them to launch the Rare Retro compilation to remind folks of those games, shop the IPs around, and then announce new games for those IPs at this year's E3.