iTWire has the scoop:
Some outlets have suggested that he doesn't care and will happily pay a fine if ever confronted by police; others, quoting Steve Wozniak, suggest that he had some kind of permit to do so.
Neither is true. In fact the truth is far simpler.
Steve (or someone close to him) spotted a loophole in the California vehicle laws. Anyone with a brand new car had a maximum of six months to affix the issued number plate to the vehicle.
So Jobs made an arrangement with the leasing company; he would always change cars during the sixth month of the lease, exchanging one silver Mercedes SL55 AMG for another identical one. At no time would he ever be in a car as old as six months; and thus there was no legal requirement to have the number plates fitted.
I'm actually surprised no one figured this out on their own. The loophole can't be that obscure, can it?