The new iPhone 4S is built on the "skeleton" of the old iPhone 4 CDMA which has separate antennas in several points and a repositioned Mute button. In the image above we have the new iPhone 4S above and the iPhone 4 GSM below and notice that the antenna is separated right next to the Mute button and it has been moved a little lower compared to its position on the iPhone 4. Because of this, many iPhone 4 cases will not be compatible with the new iPhone 4S and either you will not be able to insert the terminal into the case or you will be able to enter but you will not have access to the Mute button.
The change is minor from an aesthetic point of view, but for case manufacturers it is a new headache. Unfortunately for the iPhone 4S you may have to buy another case even though the device is as big as the iPhone 4 and looks 99% exactly the same.
