There is a plethora of information out there on host security alone, not to mention the amount of information on network security and access control. Likewise, you can have the best network security devised to protect yourself from the Evil Internet, but a poorly administered host can be an easy target from the inside. Zip or Jazz drives are a lot easier. Given that, I think there is merit to a write-once media approach. But not, as Paul and others point out, as a general solution.
Dale is right though, there are still vulnerabilities.
Personally, I like the idea of marking the files immutable myself. This way, even root can't change the content unless the machine is brought down into single-user mode. Inc. Otis Orchards, Wa. Compressed back issues are available for anonymous FTP from FTP.GreatCircle.