In the US, AT&T and Verizon, the other two iPhone carriers, both have existing account management apps. They all work similarly, allowing customers to pay their bill, track usage, and get contact information for stores and customer support.
Sprint Mobile Zone is available free for the iPhone on the App Store. [Direct Link]
MyAT&T [Direct Link] and My Verizon Mobile [Direct Link] are also available free from the App Store.