Not sure that I buy the idea of a 360 SOC in the new Xbox. It makes a lot of sense in terms of backwards compatibility or as a sub processor for Kinect stuff but sounds horribly expensive. If it facilitates the wholesale transfer of Xbox Live content to the new system, maybe it's justified? Dropping all of that XBLA content would be disastrous considering the already shaky confidence in the Arcade aspect of the service.
The Slim 360s already have a SOC(IIRC) and if the rumours are true that they are doing a small $100 360, then that would give them another reason to keep producing them. Unless the cost comes in from them putting them in the new hardware?