Shortly thereafter, as I was about to go through the process of filing complaints against AT&T with Visa and with the state consumer protection office(s), an AT&T representative contacted me. He confirmed that as of today, the PO Box ban had been retracted and the data plan sign up process now incorporated a mechanism for providing a street address in compliance with the MTSA law while allowing billing to PO Boxes.
So, erm, all's well that ends well? I can't pretend to take any particular credit for this change, but it's nice to see that a sharp, reasoned, business-focused complaint can produce results. And for whatever it's worth, everyone I've spoken to at both AT&T and Apple has been professional and pleasant. This was only ever a business issue, and I'm quite pleased it got handled appropriately and swiftly — essentially in less than two business days. There were probably some very interesting and difficult conversations over the weekend that I'm just as happy not to have been privy to.
Meanwhile, I continue to adapt to my iPad 3G. And perhaps vice versa. More to come as my experience unfolds.