I'm assuming (since you didn't say) that you're using KDE.
If you have the ShowDesktop icon in the taskbar already, then you can use
System Settings|Regional and Accessibility|Input Actions
to add a keyboard shortcut to the following command:
Code:
dcop kicker $(dcop kicker qt objects | grep ShowDesktop | head -n 1 | tr '(' '
' | awk '{ print $1; }') toggle
(from
http://ml.osdir.com/kde.devel.kwin/2.../msg00162.html)
Edit:
If you don't already have the icon, you can add it by right-clicking the taskbar, clicking 'add applet to panel' and adding 'show desktop'