Low-speed IO requires a lot of pins for all the controllers and such. These pins cost much more when they are on the high-speed chip than when they are produced on 90nm or something, and it doesn't improve them in any way.
Even in that 360 soc module, the low-speed io is segregated to an external chip that connects with PCI-E. I expect that this is what will happen in the next xbox too -- all the high-speed io is on the main chip, but there is a small, cheap external io hub for the low-speed io that connects with a high-speed link.
Depending on your definition, this either is or isn't a soc. It would appear MS would call it one.