Hmmm, first, the hud would have to be able to be turned on and off.
I do have a gps, it would be cool to have the compass up (that's all I use when I am on my bike).
Other than that general song info. artist-title, play time/position.
I wouldn't mind it if i could mount some switchbox on the handlebars to control it. Would just need play/pause, prev, next, random on/off, hud on/off
Getting handlebar controls isn't a problem, those are easy to find. It's interfacing with whatever.
the big question is what is it going to cost. You say PDA. So what software is going to be needed, etc...