Is Penetration Testing Worth it? / Schneier on Security, 15 May 2007
http://www.schneier.com/blog/archives/2007/05/is_penetration.html
"There are security experts who insist penetration testing is essential for network security, and you have no hope of being secure unless you do it regularly. And there are contrarian security experts who tell you penetration testing is a waste of time; you might as well throw your money away. Both of these views are wrong. The reality of penetration testing is more complicated and nuanced."