It is my understanding that only cases for the iPhone 4 that either say universal, or fit the Verizon and AT&T iPhone 4 with indeed fit the iPhone 4s. Many of the manufactures redesigned their cases to fit both phones once the Verizon model came out. Because of the slight shift in the silent switch and volume buttons on the Verizon phone, covers made to fit the AT&T model will not work, unless the side is opened and was not made with two separate little button covers.
I have been to numerous stores as well as called a few manufacturers; if the case is universal it will fit. Most likely the AT&T stores have all the left over models that were made for their phone, although I have seen some that look like new models due to their color. AT&T repackages their cases, so you have no idea if it is a newer universal case. The Apple stores here swear all their cases are universal for the iPhone 4 and they fit both the AT&T and Verizon phone. Assuming the 4s is the same fit as the Verizon 4, we should be good to go!
To be on the safe side, I have purchased a few different silicone cases; one from Verizon, one from At&T, and one at the Apple store; my phone is going to be dressed the first time it heads outside.
I will returns those that don't fit and keep, at least temporarily, one that does fit. If a case made for the 4s comes out before my drop dead return date, I will buy it instead.