Any customer using a qualified iPhone data plan has unlimited access to more than 20,000 AT&T WiFi Hot Spots. Previously, using the Hot Spots required a two-step authentication process that required receiving an SMS message. Now you connect your iPhone to the network at a Hot Spot one time, and the auto-connect capability is set up for you. From then on, all you need to do is walk into the Hot Spot, and you'll be automatically logged in
Update: As pointed out here and acknowledged here, AT&T is playing fast and loose with the word 'secure' in its marketing materials. The hotspot connection is not protected with WEP or WPA encryption, and there is nothing in the default setup to prevent other users in the same location from sniffing your packets as they fly by.
This is a big step in the direction that AT&T seemed to be taking when they started boldly moving into the WiFi Hot Spot market, in terms of making sure that AT&T customers can easily and seamlessly move between AT&T's wireless network and the WiFi Hot Spots. Most iPhone users can now walk into any McDonald's, Starbucks, or Barnes & Noble location and immediately take advantage of WiFi speeds and bandwidth.
For more information about the location of the Hot Spots and how to connect, visit the AT&T WiFi site here.