What are the physical differences between an iPhone 4S and an iPhone 4?
The iFixit Teardown says that it has the same front panel (i.e. same positions for the Home button, LCD, sensors and earpiece). Other sites have also confirmed that it's the same layout as the CDMA iPhone 4.

It should work with any case made for the CDMA iPhone 4, and some of the less precise GSM iPhone 4 cases (i.e. ones with large cutouts around the buttons). It should also work with any Apple bumper purchased from January on, as those were designed to work with the GSM &amp; CDMA iPhone 4.

The 4S also has 4 symmetrically positioned black strips (gaps between the antenna segments), 2 on each side, just like the CDMA 4. It also has a GSM slot, which is pretty much the only distinguishing feature between the CDMA 4 and the 4S.