We poster when the iPhone 4S was first launched, that from the outside the iPhone 4S and the iPhone 4 look exactly the same. The difference is all on the inside. However, we find now that the AT&T iPhone 4 and the iPhone 4S are different very so slightly. The only difference we find is that the silent switch and volume up/down buttons are shifted so that the old iPhone 4 cases do not fit the iPhone 4S.
The iPhone 4S is more like the Verizon iPhone 4, although there have not been any confirmations of this. So these case by be more accommodating for the button layout of the iPhone 4S. There are of course cases that work for both models. But if you had an iPhone 4 and were thinking to get the iPhone 4S from AT&T then you probably would need to think of buying a new case as well.
So not everything is the same after all!