Will my existing iPhone 4 case / bumper fit the iPhone 4S?
Some iPhone 4 cases will fit the iPhone 4S, while others will not. Early form-fitting AT&T iPhone 4 cases will not fit the iPhone 4S.
The reason why earlier AT&T (GSM) cases will not fit the iPhone 4S has to do with the placement of the silent/ring switch and volume control buttons. When the Verizon (CDMA) iPhone 4 was released it was clear that earlier iPhone 4 cases designed for the AT&T model would not fit the Verizon iPhone 4. Apple made changes to the antenna that required moving the silent/ring switch and volume buttons slightly.
The iPhone 4S has the same design as the CDMA iPhone 4. Even though AT&T, Verizon and Sprint customers will use the exact same iPhone 4S model, these older GSM iPhone 4 cases might not fit the iPhone 4S if there are individual cutouts for the ring/silent switch and volume buttons on the side.Reports have now surfaced that some secure iPhone 4 cases with front protection are obscuring the ambient light sensor on the iPhone 4S. Apple has moved this sensor a few millimeters to the side and now some older model iPhone 4 cases cover the sensor. This forces the iPhone 4S screen to dim as if it's in a very dark room. These manufacturers are increasing the gap in their cases to accommodate the new sensor position and fit both the iPhone 4 and iPhone 4S in future cases.