It’s interesting that you can’t take your unlocked iPhone 4S over to Sprint or Verizon to enable service, at least explicitly. Also, it appears that you can take your unlocked iPhone 4S over to T-Mobile (Apple says “GSM Networks” but doesn’t mention T-Mobile specifically), but only expect to use the EDGE 2G speeds. Being unlocked also means swapping out micro SIMs when travelling internationally is possible.
The unlocked iPhone includes all the features of iPhone but without a contract commitment. You can activate and use it on the supported GSM wireless network of your choice, such as AT&T in the United States.* The unlocked iPhone 4 or iPhone 4S will not work with CDMA-based carriers such as Verizon Wireless or Sprint.
If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone is the best choice. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide. To start using it, simply insert the micro-SIM card into the slot on your iPhone and turn it on by pressing and holding the On/Off button for a few seconds. Then follow the onscreen instructions to set up your iPhone.