WordPress is a very popular open source platform. While the team works on the the 2.9 release, there are few upgrades that remedy some current security faults. It is impressive that an open source solutions takes such an aggressive stand toward security.
- two tag importer plugin support has ended
- more secure file upload rights
- suppresses vulnerable PHP code areas
- solves trackback DOS attack problem
This is not the end all be all of security. You hosting environment is also important. Also evaluate your hosting provider to ensure that you site is safe and secure.