I agree with Glenn Reynolds: if this is vandalism, at least it's clever vandalism. Wired reports that activist Joshua Kinberg has rigged up an imprompto dot-matrix spray-chalk printer to the back of his bike. During the RNC protests in New York, aspiring lefty slogansmiths can text message their suggested one-liners to Kinberg's cell phone.
Update: I originally wrote "spray paint"; it's spray chalk, which falls short of genuine vandalism. Mea culpa.