This is about the coolest thing done with an iPad since, well, the iPad itself was created. A firm named Dentsu London has used an iPad to create a series of stop motion shots assembled from sequential long exposure light paintings. They created a set of 3D graphics and then programmed the iPad to show a set of cross sections, frame-by-frame, of those graphics. The cross sections were "painted" into the air with long exposure photographs, and then the photographs were all put together into a stop motion animation.
The end effect is just amazing -- don't just watch the beginning on how they did it, make sure you stick around for the film itself. It's a beautiful piece, as well as an ingenious way of using the iPad to create something exceptional. If you're really interested in these images, the creators have also put together a book featuring some of the best stills from the video. It's quite an impressive piece of work.