Apple is now selling an unlocked iPhone (http://macte.ch/68LQS). It’s an US$649, 16GB black model.
If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone 4 is the best choice, according to Apple. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM (Global System for Mobile Communications) carrier worldwide.
The iPhone 4 works with micro-SIM cards from supported GSM carriers. A micro-SIM card is a small memory card used in the iPhone 4 and other cellular devices that stores your wireless account information. With an unlocked iPhone, you will need to obtain a micro-SIM card directly from your GSM carrier.
The unlocked iPhone will work on supported GSM carrier networks around the world. Because the unlocked iPhone is sold without a contract commitment, you can establish service and use a micro-SIM card from a local carrier at your destination. Or check with your U.S. carrier regarding international roaming and associated charges