Here's another bus shelter ad with a far less humorous bent, although its design is perhaps even cooler than Science World's sneezing sign.
Amnesty International is known around the design world for fostering innovation in advertising; this bus shelter ad space decrying domestic abuse, created for the human rights organization by German agency JUNG von MATT in June 2009, is no exception. A poster for Amnesty International shows a couple standing unremarkably together, at least when the embedded camera senses you're looking at the sign. When the camera notes you're no longer looking at the poster, the image of the couple changes to a scene where the husband batters the wife. A preprogrammed delay times the image change so no observer actually misses the message.
The sign has the tag line "It happens when no one is watching," and features the Amnesty International logo.