Apple is selling "unlocked" iPhones in the U.S. for the first time, allowing owners to switch carriers to a limited extent.

The unlocked iPhone 4s (GSM) is listed Tuesday on Apple's website for $649 and $749 (16GB and 32Gb versions.) They're identical to the version sold for use on AT&T 's network, but don't require a two-year contract.

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. It arrives without a micro-SIM card, so you?ll need an active micro-SIM card from any supported GSM carrier worldwide.