I wish my camera were more powerful or that I had been somewhere where I could stop and get a good picture of the vanity plate I saw the other day. It was a Slytherin supporter. The plate used numbers to spell it all out (leading me to believe there is another one out there). 5LTH3RIN
They also had a Slytherin Class of ______ bumper sticker, and other stuff, like bands stickers. Oh well. If I see them again I'll do better.
(update: this is the car itself, but I'm certain that the plate is not visible. Keep your eyes peeled for late 90's corollas.) (update 2: I had no idea this was so popular http://www.mugglenet.com/plates.shtml)