AT&T has released a new application for the iPhone called myWireless (free) that allows you to check your data useage, minutes used, pay your bill, or even add service to your exsting plan.
If you are planning on a little overseas trip this summer, you can use this app to purchase a plan to cover you in over 200 countries, as well as plans to extend your data. Normal disclaimer: read the fine print before making a change to your plan.
To get the application lookup myWireless in the iTunes app store. It's free.