It's always been a CDMA vs. GSM issue, the iPhone hardware has been capable but the network hasn't.
That being said, in my area AT&T wasn't an option. We had it for a 2 year contract and it was terrible! No reception at all at my house, very little at my mom's, calls would drop when going around a curve or sometimes even under a tree. LOL! Yeah, we switched to Verizon and haven't looked back! While it is awesome that this has been enabled, it was never a deal breaker before because you have to have cell service before you have talk and date at the same time.