To unlock your AT&T iPhone you will have to restore via iTunes which means that you will lose your Jailbreak.
The guys at iDB have found a way to unlock AT&T iPhone by using Redsn0w.
Unlock AT&T iPhone Officially Without Losing Jailbreak
Before you start to unlock iPhone you will be required to file a request to AT&T to get your iPhone unlocked.
1. Download RedsnOw for Mac OSX or Windows and run.
2. Click Jailbreak option and put your iPhone into DFU Mode.
How to put iPhone into DFU Mode
To put your iPhone into DFU Mode launch iTunes >> Plug iPhone to your computer >> Turn off iPhone >> Hold Power and Home button simultaneously for 10 seconds >> Release Power button after 10 secs but keep pressing Home button >> A message will show that iTunes has identified your iPhone.
3. Un-check Install Cydia option and select the option Deactivate in next screen.
4. Reboot your iPhone and launch iTunes.You will get a message that your iPhone is unlocked successfully