That's weird.
I have a FAN discount account, and was able to upgrade both my lines through Apple iPhone app no problem. I think that at least those with FAN accounts should be able to use Apple's site. Not sure about those with full Premeir (corporate-paid) accounts, though.
That said, AT&T's pre-order site was sending me to the premier section and then crapping out.
Same here.
It went in fine using the apple site.
The AT&T premier site was sucking.