HUDs are already built into many high-end cars (including several BMW and Audi models, the Chevy Camaro, and even some lower-end cars like the new Mazda3). Same for voice-controlled phone and media systems. They're actually touted as a safety feature, since a HUD requires less eye movement and refocusing than a normal instrument panel.

Of course, it depends a lot on what you do with it. Moving existing displays like the speedometer and navigation instructions to a HUD can be an improvement in safety. But if it encourages more interactive phone use, that could obviously cause problems.