John S. managed to snap a picture of this geeky vanity plate, 1 UP. He writes:
I had to resist the urge to head-butt this truck to see if I would get an extra life.
... As in Super Mario Brothers. From wikipedia I learned: These mushrooms have green caps with white spots (originally orange caps with green spots). When Mario picks up one of these mushrooms, he is given an extra life... In some games, these are called "Ultra Shrooms", which replenish large amounts of health.
Ultra geeky, but I kind of like it.