Bruce Schneier of Crytpo-Gram and other fame links to a really interesting article by Marcus J. Ranum on "The Six Dumbest Ideas in Computer Security".
Schneier's blog entry: http://www.schneier.com/blog/archives/2005/09/marcus_ranums_t.html
Ranum's article: http://www.ranum.com/security/computer_security/editorials/dumb/
Both are very interesting reading for anyone that owns computers, whether or not you adminsiter them for a living.