I guess all of you have read about it. But on /. there were a couple of articles on port knocking which are very interesting. With just ten knocks, and only using 16384 ports as range you get a 140 bit combination. If combined to some S/Key or similar scheme, then it would be _very_ difficult to guess.
http://www.portknocking.org/ http://www.linuxjournal.com/article.php?sid=6811&mode=thread&order=0
