As noted yesterday, has begun selling unlocked GSM in the United States. The 16GB49 black model starts at $649 and the 32GB version costs a $100 more. An unlocked handset offers all of the features without a contract commitment and can be used on supported GSM networks, Apple notes:
If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked4 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.
Both black and white models are being offered contract-free. Wondering how the unlocked iPhone 4 works?provides this explanation:
The unlocked iPhone 4 requires an active micro-SIM card that you obtain from a supported GSM wireless carrier. Simply insert the micro-SIM card into the slot on your iPhone, connect to youror PC using the USB cable, and launch iTunes on your computer to complete the setup.