Generally what you want to do is have your internal networks gated by hardened devices communicating over VPN. Disconnection from the Internet is simply not practical anymore, but snooping and attacks can be significantly mitigated. My fine employer works in this area and sells these sorts of devices, I would be happy to email interested parties links to our whitepapers and devices. :-)
Something like this could occur if knowledgeable people gained access to the waveforms and ability to read/write breaker controls:
http://articles.cnn.com/2007-09-26/us/power.at.risk_1_genera...
Generally what you want to do is have your internal networks gated by hardened devices communicating over VPN. Disconnection from the Internet is simply not practical anymore, but snooping and attacks can be significantly mitigated. My fine employer works in this area and sells these sorts of devices, I would be happy to email interested parties links to our whitepapers and devices. :-)