I didn't know there was an option for unlimited data on the iPad. I do have unlimited on my iPhone which an AT&T rep told me is coveted because they don't offer it anymore. The downside is I can't tether with it. I'd have to give up my unlimited data on my phone in order to have tethering. Sorry for going off topic but I didn't know the iPad had an unlimited plan.
I gave up my unlimited iPhone plan to tether... I stopped using the iPhone for heavy web surfing when the iPad came out.