Same Issue - new SIM card resolved it
Had the same issue straight from the box after delivery by UPS on a preorder iPhone 5. Spent nearly an hour on the phone with Apple Care, then they transferred me to AT&T customer support, three service reps later and no fix (stupid questions like: did you change the SIM card? how did you install the SIM card and please restart your phone one more time). Meanwhile I now had no wireless coverage on my old iPhone or the new one and the CS rep was telling me to go stand in line at the AT&T store. I was really pissed, but I stayed calm (too loyal to Apple I guess). Finally headed over to my local AT&T and they got it activated on the original SIM, three hours later it started stating no SIM again for the carrier ID (restarting would only temporarily help). Second trip to the AT&T store and 45 minutes to setup a new SIM card. Its fixed now, finally. What a day and disappointment. Additionally, my phone had a nice nick in the black finish on the corner of one of the sides it was like that right as i opened the box and saw it.