This is interesting. Apparently, a new bit of malware specifically designed to attack offline computer networks was recently discovered lurking in the operating system of a nuclear power plant in Iran. The apparent route of entry is via memory sticks. Deepening the mystery, this same virus has allegedly been discovered* at one of the nuclear power plants in Japan downed by the recent quake and tsunami.
*CORRECTION - A reader has pointed out to me that I have misread the article in question. The Stuxnet virus has NOT been found at any of the Japanese nuclear facilities affected by the earthquake and tsunami. Rather, it has been found on personal computers in Japan, and officials from that country have urged public utility workers not to use memory sticks for their work, for fear of spreading the virus to essential infrastructure-controlling programs. I apologize for the error and thank the eagle-eyed critics at the Rigorous Intuition message board for their assistance in this matter.