UPDATED: Engadget has the details:
"Due to an equipment issue, some AT&T mobility customers in Texas may be experiencing authentication issues with their wireless devices. AT&T technicians are working to resolve the issue at this time, and we expect service to be restored later today. We will provide more information as it becomes available. We apologize for the inconvenience."
Bringing this back to the top as the comments were lighting up again with Texas iPhone users not getting service. Some seem to be out, some are coming back, and others are having various degrees of luck turning on/off the 3G radios.
Let us know where you are, if you're having trouble, and what fixes are/aren't working for you.