I had a look in Settings->System settings->Common appearance and behaviour->Shortcuts and Gestures->Standard Keyboard Shortcuts but I didn't see an option to reach the desktop (nor to minimize every window, which would also work)
I know I can do it at home (Ubuntu 11.04) without trouble using Ctrl+Alt+D but I can't find anything at work where I have KDE 4.5.3
Update: For now, I've assigned the following command to "Super+D" :
dolphin ~username/Desktop &
I would prefer a command which would minimize all windows, effectively revealing the desktop. Any ideas?