One of my first ideas for Glass is to act as a HUD for driving information. Especially for motorcyclists, having speed information and even rear view camera easily within your field of vision sounds like an excellent use of this technology.
Having your speedometer on a HUD instead of a dashboard does not make someone incapable of paying attention to the road.
I mean, have you seen how some people fiddle with their radios while driving? Personally, I'd be happy if at least a part of those people had the chance to keep their eyes on front while they do that. I might be biased though, due to the german speed limits and not being a driver myself :)