At home, I added it back in by copying the file from my 32-bit laptop. I found this knowlege-base article on the Microsoft site, which explains how to create a file to run the "Show the Desktop" command from a Quick Launch toolbar tool.
There is a "Fix it for me" button, but I chose to do it manually, which is really quite easy. Start a Notepad file and paste the given text into it.
[Shell]Save the file to your Desktop (or some other location, if you prefer), using the name Show desktop.scf and then drag that file onto the Quick Launch toolbar. The old, familiar icon is back and ready for action.
Note: The above works when running Windows XP Professional x64 Edition, assuming that you have the Quick Launch toolbar displayed on the Taskbar. It should work on Vista as well. Apparently, Windows 7 has a button on the far right side of the Taskbar that performs this function, but you can not move its position; in order to use the above, you would have to add the Quick Launch toolbar to the Taskbar, as it is not available by default.