Awesome idea! XD
Freeper mcmuffin was the first to put a variation of this idea into practice months ago.
As I remember him relating the story to me, he was sitting in the passenger seat of the car his wife was driving as they pulled up to a stop light beside another car which had an Obama bumper sticker on it. He motioned to the other driver to roll down the window. When the person rolled down the window, Mcmuffin told the driver that he just wanted to let him know that some idiot had placed an Obama bumper sticker on the back of his car. :)
Mcmuffin can chime in with more details if he likes. :)