I have a 25 year old rusty station wagon and to dress it up, I have a vanity license plate rim, my old college and on the front, the license plate rim is a football team. On my back window, I have six or seven skateboard or surfing related stickers.
But as for having a license plate that says something, nothing seems interesting enough for me to pay up and get one. It's kind of like a tattoo in that I have not found anything worth paying so much for that I have to see everyday.
I did have a license plate rim in back that had my law school, but people on the road gave me seriously dirty looks, so I took that one off and gave it away last year along with a baseball cap with said law school logo. 🙂