It’s interesting that you can pick up an unlocked iPhone 4S over to Sprint or Verizon to activate your service, at least explicitly. Also, it appears that you can bring your unlocked iPhone 4S  T-Mobile (Apple says “GSM Network” but does not mention the T-Mobile in particular), but only intended to use the 2G EDGE speeds. Being open also means that the micro-swapping SIM while traveling internationally is possible.

The iPhone opened with all the features of the iPhone, but there is no contractual commitment. You can activate and use it in support of the GSM wireless network of your choice, such as AT &T in the United States .* The Unlocked iPhone 4S will not work with CDMA carriers like Verizon Wireless or Sprint.

If you do not want multiyear service contract or if you want to use a local carrier when traveling abroad, opening up the iPhone is the best choice. Come without a SIM card micro, so you need an active micro-SIM card from GSM operators are supported throughout the world. To begin using it, simply insert the Micro SIM card slot on the iPhone and turn it on by pressing and holding the On / Off button for several seconds. Then follow the onscreen instructions to set up your unlocked iPhone 4s.