It seems that both Sprint and Verizon will be applying their current "world phone" policies to the Apple iPhone 4S on their networks. Sprint's phones will come unlocked, so when traveling internationally, you can pop any GSM card in the microSIM tray of the iPhone 4S, and it will hook you up to the respective network. It's not clear yet if Sprint will unlock all GSM frequencies, so you can insert an AT&T or T-Mobile SIM, or just the 900/1800 MHz bands for international travel.
The downside is that you will be using a different number than your US one, so you have to plan accordingly. Of course, you can stay with Sprint's "roaming SIM" card in, but then the international roaming charges are no picnic.
A Verizon spokesperson has commented that the carrier will apply its regular "world phone" strategy to the iPhone 4S, so after 60 days of good standing with bills and such, you can ask Verizon for the handset to be unlocked for international travel.
So far Verizon's unlock applied to the 900/1800 MHz bands, not the 850/1900 that AT&T or T-Mobile use, and it is to be determined if the carrier will keep the same scheme.
via Macworld & SlashGear