These devices can be used only on supported GSM networks, like AT&T in the U.S. and various carriers abroad. If you want to use an iPhone 4S on Sprint or Verizon, you must purchase the device directly from the carrier.
Apple says that an unlocked iPhone is the “best choice” for users who “don’t want a multiyear service contract or prefer to use a local carrier when traveling abroad.”
In a somewhat related story, a recent leaked memo indicates that Sprint will be locking the SIM on iPhone 4S devices. When Sprint first started carrying the iPhone 4S, there was some confusion over the status of its SIM lock. When locked, the SIM can only be used to roam abroad. This means that customers have to pay Sprint roaming fees, rather than buy local microSIMs for coverage needs.
Sprintfeed obtained a memo sent to Sprint stores that states that iPhone 4S devices will be SIM locked as of Thursday. The memo goes on to state that customers who purchased a device before Nov. 11 will not have their devices retroactively locked.
One of the advantages of buying an iPhone 4S in other parts of the world is that the devices are generally sold unlocked, whether they are purchased on a contract or off. Apple started selling unlocked devices in its U.S. stores last year.
The iPhone 4S looks and feels exactly like the iPhone 4.