Well, it seems that the masses have spoken, and Toyota has now announced that the proper term for a posse of Priuses is -- Prii.
I'm going to assume it's pronounced pree-eye. It was determined in an online contest by the automaker, beating out the singular plural Prius, as well as Priuses, Prium and Prien.
Hey, one cactus, two cacti -- one Elvis, two Elvi. I suppose Prii is just the natural order of things. So what happens when there's a gathering of two Nissan Leaves?