well not sure if this has been posted yet but i will give it a try
i have the USAF logo change as my signal goes down
the files are
Default_1_Bars.png - Default_5_Bars.png
(these images are the ones shown when you are in an app)
and
FSO_1_Bars.png - FSO_5_Bars.png
(these images are the ones shown on the home screen and lock screen)
make sure you back up the originals
you can find them in
/System/Library/CoreServices/SpringBoard.app
i know this is no fancy app that does it for you but its still something.
enjoy,
-Brett