Comment 30 for bug 91784

It seems the main difference between how OS X handles this and how compiz with the showdesktop plugin handles this is that in OS X when you select one of the apps that is already running it brings them all back and puts the windows for that app on top of the stack. This is a small enough change that can probably be accepted upstream. If I'm remembering correctly the main thing upstream did not want to lose was the "enable show desktop, open some windows and work with them, disable show desktop to bring all the others back" functionality.