Vity-ok and others asking a similar question, it doesn't look possible for anyone to unlock it who didn't at some point have the phone on contract with AT&T. I put my success story in yesterday earlier in this thread, and I haven't had an "active" account with AT&T since early 2010 when I left the US, but I did have an account back then and they absolutely had to look that account up in order to do this procedure.

So at least in my case, I was able to do it for an account that's been closed for 2 years since I paid the ETF back then. But there doesn't seem to be any way that they'll unlock a random phone from someone who never had an account on that phone. If this continues to be true, you'll have to try to find the original owner to get them to call AT&T to unlock it, or go the jailbreak route.