Google glass is probably about as distracting as the various LED and console displays that already exist, including GPS ones
Current devices in the car cannot cover the centre of your view with some graphics. In most cases they're designed so that it's actually impossible. Can the same be guaranteed for Glass? I'm not even thinking about notifications and apps. What about a random app crash / reboot - how will that affect your visibility while driving?
Have you seen the Glass videos? The screen is on the top right of your view.