I think a better choice is an EyeTap, or perhaps a variant with a variable-opacity LCD shutter to block external light when desired. These have three benefits over typical HMDs, like the one you linked:

• Ability to sense what you're seeing
  • For head-motion tracking, as an input.
  • For recording. (Glogging, or simply as a camera-phone substitute.)
  • Perhaps most significantly, and certainly the main reason, is the ability to allow accurate video overlay. Anything from face recognition and showing appropriate info (from PIM) when a face is matched, to overlaying an edge-detect enhanced IR view, to weapons targeting info.

    Come with me if you want to live.

• Ability to allow largely unrestricted vision by simply not displaying anything, but leaving the headset on. (Of course, some HMDs already have this.)

I think a binocular eyetap would be quite safe for walking around and such, though the designs I've seen don't have enough viewing angle for driving with the display off. The design could be adapted to larger viewing angles, at the cost of physical compactness, or one-eyed versions, as used now, would probably be adequate.

As for displaying while driving, it could actually make driving safer if used for driving-relevant info (speedometer, tachometer, and warning lights), or other info that the driver would be accessing anyway, but would be looking down for, such as HVAC/stereo controls. Even unrelated and distracting activites like SMS would be safer when looking up than looking down.