Will the iPhone 4S work on any carrier / network?
Although the iPhone 4S has all the hardware required to work on any GSM or CDMA network, the standard model sold in the US and overseas will be carrier-locked to the specific network you are purchasing it for.
Presumably, after some time, owners will be able to use iPhone jailbreaking and unlocking tools to remove this restriction -- but, as always, this comes with the risk of voiding your warranty and/or bricking your phone (though these risks are much less of a concern these days).
Apple will also be selling an unlocked model of the iPhone 4S soon, but this version will only work on GSM networks. It can also be assumed that unlocking the locked iPhone 4S will also result only in a GSM unlocked phone, as CDMA phones require carrier activation in order to be utilized on CDMA networks.