On "Security without firewalls: Sensible or silly?"

Security without firewalls: Sensible or silly?: "The SDSC has suffered only one security incident in a period of almost six years. "

Isn't "that they know of" relevant here"?

In any case, the paper has some good points; the one I liked most was that its not that firewalls are useless, but that they are seen as a solution to waaaaaay too many problems...