This.
I had the same problem with the iPad I received this morning. They gave me an 866 number (just call AT&T cs for help). I was on hold for an hour before I got to a representative who knew exactly why I was calling and what I needed to do to get my unlimited plan.
Basically, they help you activate your cellular data plan. They will ask you to choose the 25.00 plan then put in all your credit card info. Once you get your Cellular Data Number after activation, they make the change from the 25.00 plan to the 29.99 Unlimited plan. The hardest part of the ordeal was the long hold but after that, you're good to go.
HTH.