So Chrysler came out with the 300C STR8 and wanted to advertise how fast that thing is, and Y&R São Paulo were happy to oblige. Looks like (but the email does not explain this) they simply hung thick plastic posters with a wee car and tagline printed on it around the city (note: comments say that the posters are not plastic). When you view anything from the rippled plastic poster - it looks like it's going really fast. Neat.
Also, I've seen that "plastic see through the billboard" idea in Ian Hart's portfolio a year ago, though the Ford Mustang concept billboard is sadly impossible. Think big, but not impossibly big. ;)
A small poster works very well though, and is cheaper to produce, check it out inside.