This composition simulates depth of field. The left half is the normal rendering, the right half is passed through a Depth of Field filter. The filter takes a focal point, depth of field, normal image, blurred image, and depth image input, and combines them all to blur the frame based on distance from the focal point.
Despite the complex sounding nature, this composition runs at 30fps on my crappy GMA950 (!!). Enough to actually impress me, even :)
It's likely a bit difficult to see due to a small video, and vimeo transcoding. The original probably doesn't show it much better, but it'll have to do for now... :(
(depth buffer reading might make its way into the next GLTools release ;)