For those of you who plan to get an iPad and have chosen the option with 3G, Apple has given us insight into the way the signup and data usage will work. According to Apple there will be two plane of which you get 250MB for $14.99 per month or you can get unlimited data for $29.99 per month. In order to choose your plan you simply tap the screen and select your choice.
After you have signed up for the plan of your choosing and entered your billing information you simply get to use the data. If you opt for the 250MB you will receive notifications when you reach 20% and 10% of your remaining supply of data. From there you can choose to pay for more data or come back later.
If you choose to add more data you can simply tap the add options shown below and if you choose to cancel you can do it without calling anyone with the tap of the screen.
Hopefully Apple can add this sort of option for our phone plan in the near future to make the iPhone this easy to manage our plans with. The iPad with 3G ships at the end of April.