Some cars like Corvettes and Camaros have heads-up displays that project your speed onto the windshield. Iphone G3 has a download that will let you do this on any car (see video clip). I'm not sure if I like it, however. It seems more like a distraction. What do you guys think? Iphone 3G running Head Up Display App must see!! - YouTube
Seems like it would be alright. I don't like the fact that it lays on your dash. You would have to have it velcroed to keep it from moving around on corners and such.
Been using a similar app for years now when I need to check a speedo in a lifted 4x4.....uses the phones GPS to determine speed and displays it on the phone and/or the windshield in heads-up format (was free when I got it, but not sure now) DigiHUD & DigiHUD window