This is just an FYI for anyone else who may have a similar issue.
I received an email this morning from AT&T with the subject heading "AT&T Payment Failed". It went on to say, "We attempted to automatically renew your Unlimited data for 30 days for $29.99 rate plan. Please update your credit card information and re-select a plan to continue service via Settings on your iPad."
I checked my account on the iPad and sure enough, the data plan was expired and my only options were to select the 250MB or the 2GB plan. The part that really had me confounded was that my credit card information was all correct and up to date. Previous months had auto-renewed without incident.
So, ready for battle, I called AT&T. When they asked if my inquiry was related to the phone number from which I was calling, I selected the "No" option and then entered the cellular number associated with my iPad (you can find this in your cellular account settings). After a remarkably short time on hold, I was connected to a live person. I explained the email and the situation and asked if she could help sort this out. I kept my tone very polite and friendly. She looked at my account and admitted that she had no idea why the auto-renewal failed, but assured me that she could resolve the matter. First she asked me to reset my network settings, which did not work. Then she asked me to enroll in the 2GB plan, which would then allow her to switch it back to the unlimited plan on her end. That was it. She had me verify on my iPad that the account now reflected the unlimited data plan and that I was able to access the internet on 3G before ending the call.
I was surprised by how simple it was to resolve this. I didn't have to request to speak to a supervisor or yell and scream, not even once. If anyone else has this issue, try the above approach and you should be back in business in no time.
Keep in mind that I contacted AT&T the same day I got the email. I don't know how successful this would have been had I waited several days or weeks. Oh, and this also reminds me; remember to update your payment information before your credit card expires. This would obviously lead to the same issue.