That's normal. You cannot use an iPhone unlimited plan on a 4G phone.I have the unlimited plan with the iPhone 4. I've had the plan since the original iPhone and when I tried the Atrix out for a while ATT kept sending me texts saying to change to a new plan because it was 4G phone and needed to update my account to a new data plan. I went back to my iPhone but not for that reason (though it helped)
They will switch you to the SmartPhone Personal plan. It has unlimited data as well.
This plan is ONLY available for iPhone unlimited users switching to a 4G device. You will not see it as an available option if you order online.
You have to go to an AT&T store or call them to have them move you to the correct plan.