New photos of the device side-by-side with its predecessor suggest that the volume buttons and mute switch located along the side of the phone are now positioned just a few millimeters off from where they used to be. So if you've got a case that caps those buttons precisely, it won't offer the same usability as it did with the 4. You'll still be able to fit the phone in there, but the buttons will be scrunched in all funny.
This seems, well, a little petty. Why shift buttons around ever so slightly on an otherwise identical chassis? Is Apple in cahoots with the manufacturers of iPhone accessories? Did they have an agreement that something had to be different enough to warrant the purchase of a new case? Who knows?
Luckily, most of us probably aren't going to be facing this lack of backwards compatibility problem. I'd guess most folks upgrading to the 4S will be iPhone 3 users or those who've never had an iPhone at all. Those who actually do upgrade from the 4 to the 4S are probably the type who just have to have the newest thing all the time no matter what--and they most likely won't mind the burden of having to buy a shiny new case for their shiny new toy.