My favorite was one from California a number of years ago: UPURZ2. DMV was too stupid to decipher it before issuing it, tried to steal it back, owner refused to cough it up - can't remember if Big Stupid Government managed to grab it back or not.
I read a few years ago (it seemed to come from a legitimate source, but I won't vouch for it) about a guy who got the license plate "UKFA UYA" and explained it was an acronym for his organization, United Kids For A United Young America.
Read it in pig latin.
[At the motor vehicle bureau...]
Kramer: Yeah, I'm here to pick up my new plates. My name is Kramer. Cosmo
Kramer.
Clerk: Kramer.... [checks computer] All right...
Kramer: All righty...
Clerk: Sign right here, please. [hands over clipboard]
Kramer: [signs it] Okay. [The clerk hands him a manila envelope]. Thanks.
[opens up the envelope] Assman? Oh, no, these don't belong to me. I'm
not the Assman. I think there's been a mistake.
Clerk: What's your name again?
Kramer: Cosmo Kramer.
Clerk: [checks computer again] Cosmo Kramer. You *are* the Assman.
Kramer: No! I'm not the Assman.
Clerk: Well, as far as the state of New York is concerned, you are.
I remember a Texas plate: HMFIC2. Now HMFIC is an old military term (do I have to translate) but I don't see how being "number 2" could confer any status at all!