From the Apple site linked above:
Sign up on your iPad.
You can sign up for 3G data service with AT&T at any time right on your iPad. Theres no need to visit a store or call customer service. Just tap Settings and choose Cellular Data. Then type in your user information, select a plan, and enter your credit card information.
Monitor your data usage.
You can check your data usage in Settings on your iPad anytime. iPad will even let you know when youre about to reach your 250MB data limit. Youll get three alerts at 20 percent, 10 percent, and zero. With each alert, you can choose to add more data or wait and do it later. Tap Now and iPad opens the Cellular Data Plan window so you can update your data plan.
Manage your data plan.
iPad makes it easy to choose the data plan that works best for you. When you need more data, you can add another 250MB or upgrade to the Unlimited Data plan. Because you sign up for a data plan in monthly increments, you can cancel your plan at any time and then sign up again whenever you need 3G service.
So basically, your choices each month are $0, $15, or $30. If you forget to sign up for the unlimited plan if you go over 250MB then you'll pay more, but I don't know how you would forget since they'll alert you THREE TIMES at 20%, 10% and 0%. You'd have to be a moron.