How to unlock your iPhone if you have AT&T service?

No need to call AT&T. You can submit your request using an online form. Keep in mind, you have to meet the following requirements

● A current AT&T customer or a former AT&T customer who can provide the phone number or account number for the account
● The iPhone was designed for use on AT&T's network
● All contract obligations, including any term commitment, associated with the device to be unlocked have been fully satisfied
● The iPhone has not been reported lost or stolen.


Then go to the AT&T page and submit your info - https://www.att.com/deviceunlock/client/en_US/ 

In the form, it asks for IMEI number. You can find IMEI number from your iPhone's settings.

Go to Settings → General → About → IMEI(15 digits)

Happy unlocking!!!

Update (9/21/2012): I received the unlocked authorization email from AT&T with their instructions and also just to be on the safe side, so I followed few online forums.

Here are the steps:

1. Connect your iPhone (with SIM card in place) to the computer. 
2. iTunes will open up automatically or open it manually.
3. If the iPhone sign appears under the Devices menu on the right, right-click on it and click "Back Up".Wait for it to finish backing up your iPhone.
4. Once that's done, right click on the iPhone sign again and select "Restore from Backup" 
5. Pick the latest backup and click Restore.
6. (This is where I received an error saying that iPhone restore encountered an error because there is not enough space to perform restore.So I have to unplug the iPhone and completely wipe off everything from it. Then I connected my iPhone again and follow all the above steps again.)
7. So if everything goes well when you start Restoring your iPhone, a message appears to let you know that your iPhone has been unlocked.

Double happy :)