Dustin Curtis got his hands on and did short write up on Google Glass:
When you look at the screen, your eyes have to focus on something extremely close to your face, which leaves everything else in your field of vision totally blurred. This makes Glass dangerous to use while driving, for example, or even while walking down the street. Also, in order to see Glass’s navigation map while driving during the day, you have to look at a dark surface–I looked up at the roof of the car’s interior when I wanted to see the screen. This is also, obviously, dangerous.
I practiced focusing on my the surface of my glasses just now, and it is really straining on my eyes. I hadn’t thought of this hurdle to Google Glass before, but it’s a big one.