If you do a spread with thumb and three fingers over clustered windows of an app while in Mission Control, the windows will spread out for a better view.
It makes good sense gesture-wise, but clashes with the system-wide Show Desktop gesture.
[crarko adds: I tried this out with a Magic Trackpad and it does work this way. It does make sense for Mission Control, where there isn't a single Desktop to show.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110803223007827