] The Slammer worm penetrated a private computer network at
] Ohio's Davis-Besse nuclear power plant in January and
] disabled a safety monitoring system for nearly five
] hours, despite a belief by plant personnel that the
] network was protected by a firewall, SecurityFocus has
] learned.

Luckily the plant was offline due to other problems and has been since Feb 2002. But systems like this really should have better protection than what is described here.

The Register | Slammer worm crashed Ohio nuke plant net