The Apple website is selling unlocked versions of the iPhone 4S costing $649 for the the 16GB iPhone 4S, $749 for the 32GB and $849 for 64GB. The unlocked iPhone works only on supported GSM networks, such as AT&T in the U.S. When you travel internationally, you can also use a micro-SIM card from a local GSM carrier. The unlocked iPhone will not work with CDMA carriers such as Verizon Wireless or Sprint.
The website explains why you want to buy an unlocked iPhone 4S:
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.
The iPhone 4S has radios for both CDMA (Sprint/Verizon) and GSM. Many were wondering if the GSM SIM in Sprint iPhone 4s' es is locked to Sprint. If the SIM is locked when traveling the owner can't swap it out for a foreign carrier for a lower rate and will have to pay for international rates.
The first Sprint iPhone 4S united were not locked and now Sprint is starting to lock the SIM. An internal Sprint mem0 discovered by SprintFeed shows that starting on 11/11/11 “all iPhone 4S devices will have the SIM locked..the locking occurs during the activation process and is invisible to the customer.”
Previous Sprint iPhone 4S owners' who bought them before 11/11/11 SIMs will remain unlocked.