The exclusive to AT&T for Nokia's flagship phones deal was made for several models in advance
Otherwise there is no excuse for restricting the 920 to AT&T alone considering it managed to sell a paltry 600k Lumia 900 only! So Nokia I believe is forced to make this an AT&T exclusive due to contract conditions they struck when releasing the 900 for AT&T exclusively and likely it included the next flagship model as well. Otherwise no way Nokia would be crazy stupid enough to rest its fate in the hands of AT&T in the U.S considering its piss poor showing with the Lumia 900.
Bad times for Nokia ahead. 920 exclusive to AT&T and shitty promoting by ATT&T in turn.
On all other carriers where the Nokia 820 is available it will get slaughtered by the HTC 8X etc.
The only thing that can save Nokia now that we know the 920 is an AT&T exclusive is the hope that the Windows 8\RT ecosystem blasts off with consumers loving it. There's a high chance of that happening too, having used Windows 8 on a tablet I must say the UI and interactivity poop on the iPad. Nokia's soul now rests on Windows 8, Surface and Ms services ecosystem and how quick it will get popular.