It seems that the button layout is very similar to the Verizon iPhone and not at all like the AT&T iPhone. So, that means you can't use AT&T-only cases. You'll have to use cases meant for the Verizon iPhone. Some places are actually selling 4S cases, like Otterbox, but not everyone has 4S-specific cases.
No worries at all. They may get merged, but I was also looking fornthe same info.
Looks like, from what I've found, the verizon iPhone 4 cases are the ones that fit. I still can't confirm it though. Lol
From what I understand, the button on the upper left (silent switch) is lower on the new model like on the Verizon version. If your case/bumper has an elongated opening, it will fit both. If its more precise like mentioned above, it will not fit right.
Here is a good pic to compare. Looks like the volume buttons may be a hair lower as well. Maybe not enought to make a difference.
The iPhone 4s on the left, 4 center, Verizon iPhone on the right.