Welcome to the ET forums!
According to some here, the Gibberish seems to be caused by the Cingular Sims Cards.
As far as getting rid of the Cingular Screen altogether, there was a guy on another Treo forum who was doctoring "The Simpsons" screen shots and replacing the phone screen with those. The best I remember, it was loaded onto the phone like any other application through the hotsync . . I can't provide any links, but because I tried a couple of them myself, can speak for the fact that it can be done . .