Beeker Northam and colleagues at Dentsu London and BERG developed a method of light painting using iPads:

We use photographic and animation techniques that were developed to draw moving 3-dimensional typography and objects with an iPad. In dark environments, we play movies on the surface of the iPad that extrude 3-d light forms as they move through the exposure. Multiple exposures with slightly different movies make up the stop-frame animation.

This is pretty cool - definitely don't miss it: Hit play or go to Link - via Dude Craft