The Viral Factory produced and created a light sensitive or interactive stop motion animation with music from Hot Chip for the occasion.
The version on the Earth Hour site senses the light level via your computer's web cam and dictates the version of the story told. Check it out for yourself here:
The YouTube version has a built in toggle light switch so that you can jump back and forth between the story lines...very cool. i love this idea and the video is really entertaining too.