Information released by Apple on its iPhone 4S FAQ page brings to light some of the details of the device’s world phone capabilities. There will be an unlocked version, but you won’t be switching easily between AT&T, Sprint and Verizon.
The “world phone” moniker refers to the fact that the iPhone 4S is one model that can work with all three of the carriers that now offer the device. This is because Sprint and Verizon use a different network technology from AT&T – CDMA versus GSM, respectively – and those networks require a different cellular radio. The iPhone 4S includes both a CDMA and a GSM radio, so it can work on all three networks.
But as noted on the FAQ page, which you can find here, the network you activate the phone with matters a lot. When an iPhone 4S gets activated, only the cellular radio necessary for the network you’re using is turned on. Even if you buy an unlocked iPhone 4S at a premium – $649 for the cheapest, 16GB version, and $100 more for the 32GB and $200 more for the $64GB – your options are a bit limited. As Daggle points out, if you activate an iPhone on a specific network, you’re going to be subject to whether that network will allow you to switch to another one, even if you paid for the device in full. It’s in the fine print; you’ll be subject to carrier’s policies once you activate an iPhone with their network, and it may not be possible to switch at a later time.
With an unlocked iPhone 4S, you aren’t bound to a contract, but there are caveats there, too. Apple says the unlocked iPhone 4S will be a GSM phone specifically. You can also buy an iPhone 4S at the unlocked rate at Verizon with a month-to-month service plan, but it’s not clear if that phone will be unlocked. Chances are good it won’t be, so if you want buy an unlocked iPhone 4S, it’ll probably be best to check with Verizon or Sprint ahead of time before you bring it in and ask them to activate it for you to find out what your options are. Otherwise, be ready to find a GSM network for your unlocked phone.
However, given the speed with which new iPhones come out and the length of cellular contracts, switching carriers isn’t too big a deal. What’ll likely be more useful to users is the ability to use the iPhone 4S internationally. Apple says you can get an unlocked iPhone 4S that's equipped with a GSM radio and a micro SIM card slot. These allow users to buy a local SIM card while traveling and pay local rates (though you don’t get to use your U.S. phone number internationally).
It’s important to note that Apple mentions that when you buy an iPhone 4S, regardless of which network you activate it for, you’ll be able to “roam” internationally with your service and make use of foreign GSM networks. This is part of your carrier plan, though, and while some carriers offer international roaming plans, you’re going to want to purchase them before you travel. Otherwise, your phone will work abroad, but it’ll cost you quite a bit.