There is a difference. Most cases that are designed for both have a larger cut out for the mute switch and larger buttons for the volume rocker. When on a 4S the mute switch is not centered on the cut out and if the case volume buttons are clear you can see the volume rocker is off center.Why, the 4S is identical physically speaking to the 4. Doesn't seem to make sense produce a 4S case because it will work on the 4.
The volume buttons aren't moved. Just the mute switch. The iPhone 4S is identical to the Verizon iPhone 4. That's why the Apple Bumper will work with all iPhone 4/4S.The only physical change to the 4S from the 4 is that the volume buttons were moved slightly, so as long as it is not an older case that has individual holes for the volume buttons, you're fine. There is no reason to buy a case that is only compatible with the 4S as that case would also be compatible with the 4. I hope that answers your question, unless you have a specific reason that you want a 4S only case, which I am fairly certain does not exist.
Not exactly. ATT cases were already out, they just made new cases for Verizon iPhones. Since then case makers have adjusted their designs to work for both....and that's precisely what some did which is why they had cases for the at&t IP4 and cases for the VZW IP4.