Not Great News for Apple or AT&T In theory, everyone in the U.S. who has an iPhone is an AT&T customer, as this telecom has an exclusive contract with Apple. In reality, more than one-quarter of iPhones have been unlocked so that they can be used with other telecoms.

This figure comes from Bernstein Research analyst Toni Sacconaghi, who says that approximately 1 million of the smartphones sold by Apple in the U.S. last year were bought by people who haven't signed up for an AT&T plan.

Exactly how this compares with the numbers of other smartphones that get unlocked is not known, but iPhones are almost certainly being unlocked at a far higher rate than is common.

Bad news for the Two Main Players

It's obvious that this is bad news for AT&T, but it would be easy to think that Apple wouldn't care about this -- a sale is a sale, after all. However, Apple receives a percentage of the monthly fee iPhone users pay AT&T, which naturally doesn't happen when the device is being used with T-Mobile or another telecom.

Whether Apple and AT&T will take steps to make it even harder for users to unlock the iPhone is not yet known. This smartphone is already one of the hardest models to unlock.