The unlocking information in the wiki is a mess, references a post that doesn't exist, etc. Here are the steps to unlock a phone:
A. Do not turn on the phone or activate the SIM
B. Submit an AT&T unlock request as a “Non-AT&T Mobility Customer” via
https://www.att.com/deviceunlock/
C. You'll receive a verification email within a few minutes.
You must click on the link in this email for your request to be processed.
D. Wait up to 24 (sometimes up to 48) hours. You'll receive a second email, saying "Congratulations! Here are your unlock instructions:". Follow these instructions. Some phones will have an actual unlock code, some will not.
The general consensus seems to be that you can power on the phone (
without the AT&T SIM card in it) after step C, and begin installing apps, updates, etc. If you want to be extra cautious, you can wait until after you actually receive the unlock confirmation.