The systems are not perfect. Hacker attacks have gone on for years. The Pentagon is a regular target. During the World Trade Organization meeting in Seattle, hackers flooded the organization's Web site - slowing it, but not causing a crash. Hacking is not unique to computers, either.
Vandals attack phone systems and cable-TV systems.
The Year 2000 millennium bug illustrates how the public and news media can focus on a legitimate concern, but also blow the topic out of proportion. New technology always triggers fears. In the 1890s, some thought dynamite would put too much destructive power in the hands of radicals. Some fears prove to have at least some legitimacy. The important thing to keep in mind is that systems can manage threats, people can take reasonable precautions and government can prosecute vandals.