Security guru Bruce Schneier has written a rather fascinating article on password composition and cracking. Security professionals in general would be interested in this, but in truth anyone using computer systems (read: you) should read and pay attention to this article.
Interesting statistics from the article: 24% of all analyzed passwords are recovered within minutes; up to 65% are cracked within one month.
You can read the full article at this link:
http://www.schneier.com/blog/archives/2007/01/choosing_secure.html