AT&T's support web site should have the settings (select a Windows Mobile phone like the Tilt or 8125 to see the proper settings.
Your data connection setting depends on which data plan you have-- Media Net, iPhone data plan, PDA Plan, for example. The Media Net settings will usually work for either account though.
APN: wap.cingular try without the username and password and see if it works. AT&T seems to be dropping it, esp. with the iPhone data plan login. If it doesn't work, then use: username: WAP@CINGULARGPRS.COM password: CINGULAR1
server name: (whatever you like) gateway: wireless.cingular.com, port 80 server address: www.mmsc.cingular.com IP (I think) 188.8.131.52
-------------------- Lisa Gade Editor in Chief, MobileTechReview