I don't know this for sure, but I believe that the final step of activation occurs via AT&T (over the air) so no, this will not work.
I have heard of others that had ATT accounts, went overseas and upgraded firmwares and were stuck until they returned to the US within AT&T's reach to get the phone reactivated.
Also, during the activation process, iTunes clearly states: "Waiting for AT&T to activate iPhone." As it did this, I was under the impression that it was happenning out of band (OTA) and not the internet connection via iTunes.
Your best bet is to contact either Apple or AT&T directly and make sure this is going to work for you. Like I said before, I'm not fully certain of this, just posting it as encouragement to call ATT or Apple so that you are not disappointed.