One of the 1st things those of us who were upgrading from the iPad to the iPad 2 and had been grandfathered into an unlimited data plan wanted to know was how to transfer that unlimited data plan over to the iPad 2? AT&T setup this website allowing you to make the switch very easily. However, I recently found a reason to keep using that site. I gave my sister an iPad as a Chrismas gift and I also pay for her data plan. I recently wanted to switch to a different credit card for the montly billing. Although I was going to see her in the next few days and remind her to bring her iPad, I wondered if I could do it from the web? The answer is yes.
From this same site you can log in with the same email address and password that you used to setup your 3G account directly on your iPad and change your billing info, upgrade/downgrade your plan, add international roaming, and see your usage history. While it's great to be able to do this directly on the iPad, I actually prefer to do it on the web if for no other reason than I can have the site or 1Password remember my login credentials. On the iPad you have to key in your email address and password each time.
Keep this site handy for when you upgrade to your iPad 3 🙂