AT&T has released a sheet that explains the iPad 3G's data plans in more detail.
Much of the information was already known, such as the pricing tiers of $14.99 for 250MB of data and $29.99 for unlimited service. There are a few pieces of new information, though.
It mentions that each month happens in 30-day increments, and your credit card is charged automatically if you choose not to cancel.
The device will also manage the 3G data used, and will warn users who are about to reach their 250MB limit starting when they only have 20 percent of their data limit left. Another warning will come at 10, and the final will come at Zero. They can choose to add more data when any of those alerts pops up.
See the full PDF at att.com