Microsoft Store (online too, with free shipping) has the Lumia 520 on sale for $59. It is part of their 12 days of Christmas sales event and extended a month (12/13 to 1/12.) AND they just extended it into February.
So I ordered one. Went to the AT&T Store and picked up a SIM. The day it arrived I put the SIM in. One hour later I used their online Unlock Request Form (http://www.att.com/deviceunlock/) and 60 hours later received via email the Unlock Code (yes, the code unlocked it just fine.) Note that when you fill out the form, you need to put the IMEI number in first. If you put the phone number in first it errors out because the number has not been in service long enough to allow unlock.
The "rules" are an establish customer needs to have a 100% owned phone connected to the AT&T network for 60 days and for a pay-as-you-go customer the wait time is 6 months. Apparently because I bought directly from Microsoft who is buying Nokia (bought directly from the manufacturer) those times were waved. My phone had been connected to the AT&T network for less than 72 hours.
FYI, I am an established customer and added the SIM to an existing account. So someone else will have to try with a goPhone SIM and see if it works as well.