The iPad 3G should arrive on pre-orderers doorsteps tomorrow, and TUAW got a screen shot of the data plan sign up screen. As you can see above, it works with typical Apple simplicity. You access the data plan sign up from the settings menu.
Here’s more from TUAW: “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.”