Just finished watching a Frontline show discussing cyber-terrorism. Richard Clarke was a prominent interviewee. If you happen to have a chance to watch the show, I would suggest doing so. The show made a point of saying that the man who was right about al-qaeda is now sounding the alarm on this issue.
The show also pointed out that the DOD computers were being hacked over a period of time a few years ago. Apparently the incident was traced to a mainframe in Russia, but Frontline said that didn't necessarily mean the attacks originated there.