I had called them about that yesterday night and the lady had to put me on hold and talk to her supervisor.
The double data plan states double your data on your 4g smartphone. iPad isn't a smartphone so doesn't apply (I had to ask in reference to other 4g tablets since the ipad 3 isn't official yet)
Give them a call and try yourself but something tells me they might extend that to the ipad 3 when it is officially released to compete against ATT.