Which iPhone 4S Cases actually fit?
Cases that are cut for specifically the iphone 4s or the CDMA version of the iphone 4 should fit the 4s.

Some iphone 4 cases will fit the original (gsm) version of the iphone and the CDMA version (the cut away around the mute button is large enough to support it in either position, basically). These should also be fine.

I've got an iphone 4 "bumper" from Apple that's been cut for both versions of the iphone 4, and it fits my 4s just fine.