It looks like AT&T might be using the 802.21 protocol to relax the pressure put on their 3G network by the iPhone. The new iPhone 3.0 OS software contains the ability for iPhones to autoswitch from 3G to Wifi when an AT&T Wifi network is in range, without user intervention.
Today AT&T made a statement saying they would allow "seamless transition from their 3G network to any one of their 20,000 Wifi wireless hotspots". While the IEEE protocol wasn't stated, it is likely the way that the iPhones will switch back and forth between 3G and Wifi.
There is always the possibility that they are doing "dumb handoffs" as well which would just be like when a normal iPhone
