I have to agree with the poster above that your case doesn't hold water. This is Apple we are talking about, who of which hold a respectable reputation for their refurbished products, and I highly doubt that they would ever take a returned/repaired phone without re-flashing it, setting it to factory settings; it is hard to believe the phone came "unlocked."
We all know that ZiPhone is a great program, but is known for causing problems (go check out the hackint0sh forums). So to sum up your case;
1) You need an AT&T card in order to bypass the activation screen, therefore if you do not have an account and you somehow are on the mainscreen, you have therefore, and obviously, did a 3rd party modification, which voids your warranty or rapport with Apple
2) Even if the iPhone came locked, you unlocked it eventually, and you had the problems you're experiencing.. you eventually end up in the same situation, your phone is hacked and you dont have an AT&T account, they have every right to show you the door
3) Seriously, again and again, your phone coming "unlocked" doesn't fly. This isn't BestBuy where they accept iPod returns that have engraving on them and turn around and sell it, Apple refreshes their returned products to almost like-new state, which also means software
