It's not confirmed by the telecom giant, but several people have been able to replicate it: If you own an iPhone, you can log in at an AT&T hotspot or a Starbucks through their AT&T portal link (upper right corner of the T-Mobile screen, and soon to be a unique SSID), enter your phone number, and have free access. This is nifty, and not unexpected. AT&T is providing free service to 7 million DSL and fiber customers and 5 million remote business access customers. Adding a couple million iPhone users as an additional tie for continued loyalty is a no brainer.
Update: Well, that didn't last. It was clearly a test, only a test, and AT&T has pulled the plug for the moment. I suspected that with no announcements, and a national network, they might simply have flipped a switch not understanding how rabid we are all.