http://www.techcrunch.com/2008/06/11/my-blog-was-hacked-is-yours-next-huge-wordpress-security-issues/
Article on WordPress security by TechCrunch. Good post encouraging people to keep their version up to date, etc.
http://sourceforge.net/projects/tripwire/
Tripwire monitors files on the filesystem for changes as a method of intrusion detection. Similar systems include AIDE and Samhain. Useful for detecting web site hacks.
http://ocaoimh.ie/2008/06/08/did-your-wordpress-site-get-hacked/
Donncha writes a useful article on detecting if your site has been hacked, monitoring systems to check for hacks, and how to secure your site to avoid future hacks.