After help my my good friend on the iTunes team, we determined that my account was still registered as a corporate account, rather than an individual account (iPhone only works with individual accounts). This was the result of me being a customer for more than 10 years.
I called AT&T back, and the CSA very quickly took some action to somehow finalize the conversion of my account from business to individual. Apparently, this final step had not been taken by the previous CSA. As soon as she did that, I was able to activate my phone, and my number ported within the hour (probably faster; I didn’t check it until an hour later).
Thanks to everyone (especially Mike W.!) who helped set this mess straight.
So far, I'm blown away by the phone, but there are some peculiarities. First and foremost, I wish all list views could have entries deleted by gesturing, rather than having to enter edit mode. Perhaps Apple will refine this sort of thing soon.
More to come, I’m sure…