A USA Today article discusses the practice of locking cell phones to providers. Apple's iPhone, for example, is "locked" to AT&T and can not easily be used with other cell phone providers.
AT&T's Jim Cicconi offers an interesting comment that AT&T will "gladly unlock" a customer's phone once the contract has been fulfilled. Apple's iPhone requires a 2 year contract with AT&T.
[ via iPhoneCentral ]
Update: TitoC points out that in the sidebar, the iPhone is listed as an exception to the unlocking offer:
AT&T will unlock phones for customers once they have fulfilled their contracts, which typically run one to two years. One big exception: Apple's iPhone, distributed exclusively in the USA by AT&T. "That's different," says AT&T spokesman Mark Siegel.