Security: Open source vs. Commercial
Security guru John Viega confronts the myths surrounding the security of open source software in his recent article: Open Source Security: Still a Myth. With a title like that, you might want to read through the hullabaloo in the comments at the end of the article.
There's only two differences that open source brings to the security equation... First, because the source code is available there is the possibility that security problems may be recognized by one of the more eyeballs. Second, because the source code is available there is no place for the developers to hide when a security problem is found.