Apple sells unlocked iPhones in U.S.

Apple on Tuesday began to sell unlocked iPhone 4 GSM models to customers in the United States from its online store. The phones cost $649 for a 16GB model, $749 for a 32GB model. Black or white models are available.

"Unlocked" iPhone 4s arrive without an active micro-SIM card in place. Customers must first obtain a micro-SIM card from a supported GSM wireless carrier before the phone will work.

The unlocked iPhone could also work on T-Mobile's U.S. network, which operates using GSM like AT&T. But because of differences between the carrier's networks, an iPhone 4 operating on T-Mobile's network will only work using slower EDGE data technology, rather than at 3G speeds.

This new unlocked phone is primarily useful for U.S. iPhone customers traveling internationally who want to use local carriers rather than incurring "roaming" fees from AT&T.

Unlocked GSM iPhones are a great deal more expesnive than phones purchased through AT&T. Buying a GSM phone through AT&T costs $199 for a 16GB model, $299 for a 32GB model. But doing so also requires the customer to agree to a two-year contract that incurs large fees if the customer terminates the contract early.

Customers in many other countries have been able to purchase unlocked iPhones, in many cases, depending on the law, right from the start. But unlocked phones are still something of a rarity in the United States, where cell phone carriers exercise much tighter control over how cell phones are sold and distributed.