This was published a while ago, but on The Smoking Gun there's an entertaining list of complaints to New York's Department of Motor Vehicles about obscene vanity plates. Some of the examples are pretty creative, but it seems that whether you use numbers instead of letters, put it in a foreign language, or write it backwards, someone out there is going to be offended by your suggestive plates. My favorite is the letter to Gov. Pataki, arguing that the writer's plate should be permitted: "In this war on terror, USKIXBUT is needed now." Yeah! That'll show Osama if he's stuck behind you in traffic!
As for me, the worst license plate I ever saw was GOATSE. Why anyone would put that on their car is beyond me, but I imagine the advantage is that any nerds who understood the reference would be too jaded to complain to anyone.
This sort of plate, on the other hand, is simply disgusting.