The people at the Apple Store said that they have been having issues since Friday with the Att server being very slow with all the upgrades the last couple of days.
iTunes activations go to Apple servers, and from there to AT&T servers. The webpage I gave you cuts out the Apple in the middle part. I suspect that the logjam is at Apple, not at AT&T, which is why I suggested you try activating directly with AT&T.
It may be faster, it may not be, but it's worth a shot. Be SURE that you back up your existing phone first.