Setting up your 3G service on the iPad 3G
As you can see, you simply touch the Settings iPad app, tap on Cellular Data, enter your payment details, and boom! you're presented with a Cellular Data Account page which shows you your Cellular Data Plan account number, the status of your plan (how many days you have remaining), the billing period, and the abilities to add data or change your current plan and also add an international data plan.
Robert also notes that AT&T apparently assigns you a new mobile number for billing purposes.
We can imagine AT&T will be inundated with signups this weekend, so let's hope their servers and data towers are up to it.