Sound. Specifically when in background mode, although it might also be useful in the foreground. I'm guessing that it should be configurable.

I'd like to suggest 2 different beeps or tones. I'm thinking 1 tone to let you know when you're within 1.5miles of the next waypoint, and 1 to let you know that you've deviated .5 mile off route. I figure that this is going to be most useful for long trips when having gpsapp active for 30 miles of interstate isn't very productive and having the player visible is going to be more useful.

Actually, the option of using short .wav files would be cool. Then I'd have to trawl the web for clips from Knightrider, "Michael, we're going the wrong way."