Well, to be more accurate, free for AT&T iPhone users. If you have your service through AT&T still, then you should get this login page when you connect to the attwifi SSID at Starbucks, or anywhere else. Enter your 10 digit AT&T mobile number and you’re online.
Devicescape users can add the special ‘AT&T Wi-Fi (iPhone)’ credential type to their account, and they will get online automatically.
There have been rumours around that the same will be enabled for anybody with an AT&T mobile data plan at some point; I expect the iPhone is being used as the test platform here since they already had mobile pages for the iPhone (my N95 still sees the full page).
Update, May 4, ’08: Seems that the web page has been taken down now (guess too many folks were abusing it by changing their browser agent string and getting online with non-iPhone devices).