The new 3.0 firmware now supports auto-authentication. Just sign in once to an AT&T wifi hotspot and you will then seamlessly jump from AT&T cell towers to AT&T wifi. Awesome! Here’s the info from AT&T:
Seth explains that AT&T will support auto-authentication for iPhone OS 3.0 users connecting to AT&T Wi-Fi Hot Spots. Auto-authentication allows iPhone users to seamlessly switch from AT&Ts 3G network to an AT&T Wi-Fi Hot Spot without being prompted. For more information, visit http://www.attwifi.com