:)
I'd never even thought of this before until you asked. 15 minutes of Googling netted me these search results:
http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&q=%22show+desktop%22+script+command+line&btnG=Search
Nothing directly answered the question, but a little bit of head scratching later, a hacky way of doing it came to my mind. Make the simple script that's mentioned in many of those links, name it [whatever].scf (I just called it SD.scf), drop it in the system32 folder, and then it's available from your command line. Just type [whatever].scf (remember to include the extension .scf) and it works!
Is that good enough?
[edited for clarity]