I think the people who call them Prii are showing off their knowledge of Latin, perhaps in a light-hearted way. We have friends whose last name ends in -us and when we refer to the whole family, we pluralize them with an "i," just to be silly. I might say to a friend "You see those cars everywhere now - soon the whole nation will be driving Prii," but I would never call the car dealer and ask if he had any Prii on the lot.
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Anonymous2 weeks further down the road of life and much reading leads me to find that Prius comes from the Latin for "before it's time" so the correct plural, I suppose would be Prii