AT&T app is built-in as firmware update. You must have missed the update where they updated the logos and making the signal bars appear to be twice as much as actual signal.
This looks like a very useful app, containing features that should have been included natively. I know not everone has AT&T, but it could have allowed some variations for each carrier (as they all bill for the same thing: minutes and texts). Too bad AT&T didn't make one, but I guess it wouldn't make too much business sense for users to be aware of their minutes/texts used (until billing time - just shows you what kind of company they are).
AT&T would rather have you worry about going over minutes/texts after you have. In fact, I bet they have already done a study on how much lost revenue they would experience if a customer had insant access to their phone useage.