since this thread has only recently been unearthed again, here are a few around these parts:
a honda element with HELIUM (seriously)
a suburban with SRUSLY
a VW eurovan with NO MAP
a coworkers old nissan had plate WTF and constant offers to buy it.
my grandparents (now both passed) always had new hampshire PING and PONG.
I admit to having a vanity plate. couldn't resist, as they're a one time fee of $30.
My 89 toyota corolla wagon's plates are FLYWHP. picture is on home computer.
personal theory on vanity plates: more common where they are cheaper. like here, or new hampshire. it's really easy to make a one time payment of $30 for a bad idea.