Using the Windows Keys for shortcuts in Gnome

Alt least on Ubuntu, the default keyboard configuration for Gnome does not allow the assignment of shortcuts using the windows keys. The keyboard shortcut dialog (System/preferences/Keyboard shortcuts) recognizes the windows-keys als "Super L" and "Super R", but it does not allow to use these keys in combination with other keys (e. g. win+D).

To get rid of this limitation, go to system/preferences/keyboard. At the "Layouts" tab, click on "other options", and check the option"Alt & Win key behaviour/Hyper is mapped to the Win-keys". Done!
The keyboard Shortcuts dialog now recognizes the windows keys as "Mod4", and you now should be able to compose keycombinations using those keys.

By the way, another killer tool concerning keyboard shortcuts is Gnome DO. It provides a kind of quick launch feature (similar to spotlight on Mac OS X) that lets you select and launch programs in a very effective way.