The rep was astounded that I couldn't use the card for AT&T. They had never heard of such a thing. (Which confirms I'm not just mistaken here.) They searched their internal database and found no references to disallowing PO Boxes as billing addresses. After discussing it with them and doing some research on the Visa site, I'll be calling in tomorrow during business hours to lodge a merchant complaint against AT&T. Per the Visa site, I am encouraged to notify my credit card issuer of "any merchant practices that you feel are inappropriate." Not honoring a card with a legal address and no evidence of fraud certainly counts as inappropriate. I'm also planning to call Oregon's consumer affairs department, once I figure out which one (there are several) and lodging complaints against AT&T there as well.
In the department of minor ironies, I was trying to view one of the Apple guided tour videos on the iPad section of apple.com, to see if I'd missed anything about how to use Safari on iPad. The lack of tabbed browsing is a real issue. It took about ten minutes for Safari on iPad to stream me the 90-second video about how easy it is to view videos on Safari on iPad. I watched in 5-7 second blocks, repeatedly restarting. Um, yeah. And they say irony is dead.