One of the best things about the iPhone 4S was that it rolled GSM and CDMA into the same baseband. What that means is if you have, say, an AT&T iPhone, it could technically run on Verizon as long as you unlocked it. Likewise, if you had a Verizon iPhone, you could just slap in a GSM pay-as-you-go SIM card if you were traveling in any other country in the world to avoid exorbitant international roaming fees.
With the iPhone 5, though, things are changing. There will be two versions of the iPhone 5, a separate device for both GSM and CDMA carriers. Why? Seems to all be about LTE.