So, just for giggles, I tried to unlock my iPhone 6+ which still has nearly $600 left to pay... sure enough, I just got this email. So I can only assume YMMV.
Unlock request number: <omitted>
Thank you for contacting AT&T about unlocking your mobile device.
Congratulations! Here are your unlock instructions:
1. Open iTunes on your Mac or PC and verify that you have Internet connectivity.
2. Ensure the original SIM card that came with this device is inserted in your iPhone.
3. Connect your iPhone using the dock connector to USB cable that came with your iPhone.
4. Backup and restore your iPhone using iTunes. For information on backup and restore, please visit http://support.apple.com/kb/HT1414.
5. After restoring, your iPhone will be unlocked.
Additional information on unlocking can be found at http://support.apple.com/kb/HT5014.
Questions? Please call 800.331.0500, or dial 611 from your AT&T wireless phone.