Aayush Arya writes at The Next Web:
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.
All that for a license plate! How about that, huh?
As to why he went to all this effort in the first place, Walter Isaacson noted in his biography of the Apple co-founder that Jobs may have wanted a numberless license plate to prevent himself from being tracked — but when asked in interviews after the book launched, the author has not clarified this statement.