Pip has created the ultimate geek experience: the TLA number plates pool on Flickr. They're mostly file extensions and other acronyms known to geeks, but quite amusing.
When I was a kid, we had a game to pass the time on long journeys, where you had to make up words to go with the letters of a number plate, e.g. WHW might be World Heavyweight Whistler. Maybe you could get extra points for spotting a number plate that already is a three-letter acronym.