As of April 8, AT&T is unlocking iPhones. Now, it won't unlock just any iPhone; there are stipulations you'll have to meet before being freed from the carrier.
The 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."
If your account and device fall within those guidelines, you can request an unlock. According to The Next Web, it's a fairly painless process.
First you'll need to get the IMEI (International Mobile Equipment Identity) of your iPhone. You can find this by going into Settings > General > About on your device.
Next, contact AT&T. You can do this via online chat or by calling customer care. Let them know you'd like to unlock your iPhone, and provide the representative with the IMEI you found in step one.
Wait patiently for an e-mail from AT&T with instructions on how to complete the unlock process. The process requires you to connect your iPhone to iTunes and then back up and restore your device. You will lose your jailbreak during the process.
You can unlock up to five devices per year on your account. Once your device is unlocked, you're free to use your device on any GSM carrier you'd like. Use your newly unlocked iPhone on T-Mobile (without 3G, mind you), or take advantage of a prepaid international SIM while traveling abroad.
If you're a Verizon Wireless customer and you'd like to get your iPhone 4S unlocked, you can call and request it from customer care. If your account's in good standing, and you haven't requested an unlock code on that line of service in the last 10 months, you'll be granted the unlock. Once your device has been unlocked, you'll be required to insert an international SIM to complete the unlock process. AT&T and T-Mobile SIMs are not supported (I've tried). For more information on the unlock policy and process, you can view this Verizon Support Forum post.
Sprint customers, you, too, will have to. Once it's approved, you'll be required to place an international SIM in the device to finish the unlock process. As with the Verizon model, you won't be able to use an unlocked Sprint iPhone 4S on T-Mobile or AT&T.
Have you already called AT&T and requested an unlock? If so, how did it go? Let us know in the comments below.