I’ve been using Ubuntu/Gnome fulltime now for some 2 years I believe. I never found out what the “show desktop” shortcup was. I always used the little icon. But while I was tweaking compiz I found out that the shortcut was d. I Windows it was Super+D (Windows key + D). So I decided to change the keybinding to that. I couldn’t manage it via the “Keyboard shortcuts” configuration. I kept insisting on Super_L. So I “hacked” it via the gconf-editor. See the screenshot for the correct value:

But, I realized I had to set this up every time I reinstalled Gnome/Ubuntu. So I changed it back to Ctrl+D. Convention over configuration maybe?