I just posted a silly little sample app called WindowFlipper. It contains code that replicates Dashboard's "flip over a window to see its back side" effect. I'm doing all of this manually (NSAffineTransform seems to only let you rotate and scale stuff), so it isn't blazingly fast, but maybe it's of use to someone.
For what it's worth the actual code is in a category on NSWindow, so you should be able to copy this and use it fairly painlessly in your own apps. Just remember to credit me and I'll let you have it for free.