At least one carrier has posted details in the form of a PDF about how their plan will work.
The quick sound bite:
Recurring monthly charge with no long term contract
• $14.99 per month for 250 MB
• $29.99 per month for unlimited data
• Unlimited access – no added cost – to AT&T’s 20,000+ Wi-Fi Hot Spots
What happens if I exceed the amount of data in my selected plan before the end of one month?
With the on-device management system, you can check your data usage in Settings>Cellular Data on your iPad at anytime. Also, for the $14.99 plan, iPad will even let you know when you’re about to reach your 250 MB data limit. You’ll 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.
If you do exceed the amount of data in your plan, your plan will expire, but it’s easy to add another one. Also, with domestic plans, you can wait until the 30th day from your purchase when your plan will automatically renew going forward.
Apple has their own web page to explain how the 3G iPad will work.
It will be interesting to see how other carriers handle their plans.