You can cancel the auto-renewal of your AT&T 3G iPad data plan at any time. Follow these instructions to cancel your service:
1. On the iPad navigate to Settings -> Cellular Data -> View Account
2. Log in to your AT&T Cellular Data Account with your email and password and touch Next.
3. Select Add or Change Data Plan -> Cancel Plan
4. If you want to reactivate your data plan within 60 days you can select Cancel Plan and Delete Account Later to avoid setting up the account from the beginning.
5. Choose Cancel Plan and Delete Account Now to remove your account information from the iPad immediately.
6. Touch the Continue button.
Your AT&T iPad 3G data plan will no longer automatically renew at the end of this billing period. You will continue to have 3G service on your iPad until the plan you have already paid for runs out or you hit the data limit.