I don't have a PDA yet so I'm lurking for now but I might have a idea for tactile "bumps" that also glow in the dark. Try using a toothpick to place a single dot of paint on the protective sheet for each button. You could use a braile (sp?) like pattern ie. one dot, a triangle, a square etc.
Can you photocopy or print a screen shot of your layout? That might help with bump placement.

Also, how do you get the overlay to stick?