I think a lot of people are looking to upgrade their older models. I still have a launch 20gb 360 Pro which I've somehow managed to keep alive, but I'm really considering picking up a 360 S so that I can retire my current 360. Most of my friends took advantage of the GameStop deal that let you trade in your old 360 for credit towards the 360 S.
I'm willing to bet that a large chunk of the 360 S sales are from existing 360 users. Very different from when the PS3 Slim launched, because most existing PS3 users really had no incentive to purchase a Slim. For example, I also have the 80gb PS3 with BC, so there is really no need for me to upgrade, and I'd imagine that most other PS3 users felt the same way. The PS3 Slim launch probably increased the actual PS3 user base more than the Xbox 360 S did for its user base (where a lot of the sales are from people upgrading).
