Chalk this up as a win for anyone with a GSM iPhone: AT&T on Friday announced that starting Sunday, the company will allow customers who have fulfilled their two-year contracts to unlock their iPhone for use on any GSM network.

AT&T gave the following statement to 9to5Mac (and confirmed it in an email to Macworld Friday): “Beginning Sunday, April 8, we will offer qualifying customers the ability to unlock their AT&T iPhones. The only requirements are that a customer’s account must be in good standing, their device cannot be associated with a current and active term commitment on an AT&T customer account, and they need to have fulfilled their contract term, upgraded under one of our upgrade policies or paid an early termination fee.”

This brings AT&T’s iPhone unlocking policy closer to that of its other supported devices: In the past, subscribers could unlock a non-Apple phone after their contract had expired, but the company refused to unlock the iPhone except in special circumstances.

Now, customers who wish to use AT&T’s network can purchase an iPhone at a subsidized price and have it unlocked after their contract term is up, rather than purchase an unlocked phone at full price directly from Apple.

Unfortunately, AT&T subscribers who wish to roam internationally on their iPhone 4S using a local micro-SIM card will still have to purchase an unlocked device at full price: AT&T won’t unlock the GSM micro-SIM for international use during a contract term, as doing so would also enable you to use any GSM network at home. In contrast, Verizon and Sprint can offer the iPhone 4S with the GSM micro-SIM slot unlocked for international use because the companies use the CDMA standard in the United States.

If you have an out of contract AT&T iPhone 4 or 3GS, however, the stakes change: It’s now a lot cheaper to use it internationally, or give it to a friend or family member who uses T-Mobile.