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It's the black text on a bright background.
The reason it takes so long for your eyes to adapt to dark conditions is level of neurotransmitters in your eyes. When you sit in the dark, the nerves aren't firing so the neurotransmitters build up, making your eyes more sensitive to light - better able to see in the dark. This is also why going from a dark room to daylight hurts your eyes - your eyes' heightened sensitivity makes everything seem much brighter.
Anyway, if you look at something significantly brighter than your surroundings, it depletes the neurotransmitters in that area of your retina, leaving an area of reduced sensitivity. This phenomena is commonly known as a sun spot, but can be triggered by not only the sun, but other relatively bright lights such as a camera flash. The key is that brightness is relative. Shining a cheap flashlight in somebody's eyes at night can leave them with a sun spot, while they may not even notice that the light is switched on during the day time.
Getting back to the topic at hand, looking at your LCD backlight isn't going to instantly give you a sun spot in your vision. It will however leave an area of reduced sensitivity. The brighter it is and longer you look at it, the greater the reduction. This means that when you look back up at the dark nighttime road, you won't be able to see as well in that key center area of your vision.
Reducing the display's bright area reduces the impact on your nighttime vision. The simplest way to do that is to use a transmissive type display... Lit text on a black background.
Transmissive LCDs have daytime readability issues due to their design and VFDs have power consumption issues. OLED displays have neither of those issues, and therefor should an the ideal solution.
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