A truck-mounted device spits a row of paint dots under computer control. Drive along the Interstate while printing out political diatribes. Make our highways look like a Newsgroup. Use high-pressure water sprays instead of paint for temporary, less illegal road gibberish. Can you be arrested for CLEANING little spots on the highway? Colored dyes would work well on packed snowy roads (a little carbon copier toner or fluorescene dye goes a long way.) Rent out ad space in fields near airports, then do your printing in seeds for variously colored crops.
The "Cyranoid" idea is quite thought-provoking as well.