Called AT&T to see what they could tell me. They had me change a setting under Connections > GPRS > AT&T IMS > Access point = wap.cingular
The original setting there was ims.cingular.
Works fine now. Just an FYI: the help desk person did say that "sometimes" they have an issue with being able to send email with that setting. He had me send an email (POP3) and it worked, so he said it should be fine.
iPhone 3GS, iPad, 27" i3 iMac, AppleTV, AirPort Extreme