Today AT&T's very own application, myWireless, was released in the the App Store for all AT&T subscribers to download. myWireless [iTunes Link] is a crafty little app that lets iPhone users manage their AT&T accounts easily from the device itself. You can view your data and minutes usage, view and pay your bill, and you can even add and remove extra features like text messaging and early nights/weekends, all from the comfort of your very own iPhone.
I've tested it out and pretty much any option on AT&T's website is built into this app. You can even access your full detailed bill via PDF file.
It is important to note that you must have an active AT&T Mobility account that is registered for online account management in order to use this application.