I thought AT&T did away with the user name and password. My son's iPod Touch always connects and has never asked for anything when connecting to AT&T hotspots.
You just need to check the "I agree" box if a screen comes up. However, if I remember correctly, this has only been the first time. All AT&T wifi hotspots are configured the same so as long as you have "Remember Network" selected as Yes, it should never ask you again and just automatically connect.