I think the Chinese and the terrorists are putting considerable effort into this.
This article is from 2002 and cites specific instances of attacks, not just plans:
Cyber-Attacks by Al Qaeda Feared
http://www.washingtonpost.com/ac2/wp-dyn/A50765-2002Jun26
Late last fall, Detective Chris Hsiung of the Mountain View, Calif., police department began investigating a suspicious pattern of surveillance against Silicon Valley computers. From the Middle East and South Asia, unknown browsers were exploring the digital systems used to manage Bay Area utilities and government offices. Hsiung, a specialist in high-technology crime, alerted the FBI's San Francisco computer intrusion squad.
Working with experts at the Lawrence Livermore National Laboratory, the FBI traced trails of a broader reconnaissance. A forensic summary of the investigation, prepared in the Defense Department, said the bureau found "multiple casings of sites" nationwide. Routed through telecommunications switches in Saudi Arabia, Indonesia and Pakistan, the visitors studied emergency telephone systems, electrical generation and transmission, water storage and distribution, nuclear power plants and gas facilities.
was that an intrusion - or just information available on the sites that they were looking at?