International travelers who want an iPhone 4S for its global roaming should buy their handset from Sprint or Verizon Wireless. These two carriers will unlock your phone so you can use it with any international rate plan. (Updated below)
It all comes down to the micro-SIM slot and what each carrier is going to do with it. AT&T locks the micro-SIM slot so you can only use the phone on its network. When you travel overseas, you must purchase AT&T's costly international roaming plans.
Sprint and Verizon, though, use CDMA for their domestic service and the micro-SIM slot for international service only. These two carriers can unlock the micro-SIM card slot and not worry about people jumping ship to another carrier.
Between Sprint and Verizon, Sprint offers the better deal on the iPhone 4S. It's selling the handset with an unlimited data plan and an unlocked micro-SIM slot. Verizon does not have an unlimited data plan and will unlock your handset, but only after you have been a customer in good standing for 60 days. This international unlock is available for all Verizon's world phones, but this is the first time it applies to the iPhone. [Macworld]
Update 10/12/11, 1:40 PM:
Ars Technica reports that MacWorld didn't exactly have the story right. According to a Sprint representative, Sprint customers who buy the iPhone 4S will not receive unlocked phones that would allow them to pop in local microSIM cards while travelling abroad. Like AT&T, Sprint plans will now support international roaming. The Sprint representative suggested that Verizon's iPhone 4S will work the same way. —MA
Update 10/13/11
Macworld has an official statement from Sprint that clarifies its unlocking policy for the iPhone 4S. Initially, the iPhone 4S will be sold as an unlocked handset. Shortly after launch, Sprint will push out an update that will lock the handset. Customers in good standing that are travelling overseas can request an unlock when needed.