I can only pray our future is this obnoxious. A media group in Berlin hacked together a sort of digital slingshot. Type in a message, aim at a building and fire. Your message splatters all over a facade. There's video:
While a projector handles the actual imagery on the building, of course, the slingshot itself is more than a mere wooden prop. Equipped with Arduino, a laser, communications radio and keypad, it truly interacts with the projection system (or in other words, your aim matters.)
Now the only thing we need is several billions of dollars in projection infrastructure, connected through another several billions of dollars of easily hackable network infrastructure. Then - hope you're still with me here - the building taggers of the world can replace crummy old spray paint with digital graffiti.