Launchpad uses a blurred version of your Desktop image for its background. Hitting Command+B while in Launchpad cycles through the original image, the blurred one, a black and white version and a blurred black and white version.
[crarko adds: A couple of people mentioned this. I don't necessarily see much use for it, but apparently one of the Launchpad developers thought it was a nice touch.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110918172310363