Government (at the US) is the one that sets the security ratings of operating systems. Our government uses linux in applications where it is needed. For instance for national security. open source yet no one knows what open source project they are based upon. That in itself makes it hard for someone to be able to setup an attack against it. Altho for desktop systems at a local school. dells with windows xp(i hate those things).

The main reason people critisize open source is because it is beyond their knowledge. I am learning programming right now. I will be the first to tell you I don't know shit! You can't critic open source if you don't know source!

Lastly, most security flaws are found by geeks. If a lot of geeks can look at source it is less likely that something will be exploited. As with closed source once a security hole is found and exploited it will be ongoing until it is patched.