Now that's a neat Idea, you could be a billboard when the time is right and take it down at other times... You might even get more creative and hook up your smartphone or tablet as a display on the other side with whatever wording you want.

Maybe a scrolling marquee app. Heck, you could advertise whatever you want on your display at that point.

If it then becomes a trend, one could pay people small amounts of money for becoming mobile billboards on location enabled devices.

There's also those window stickers that adhere to a lot of things but peel off easily.