According to a recent panel discussion at the AAAS with Richard Clark, previous adviser for cybersecurity under George W. Bush, the US government and businesses are ill-prepared to defend vital infrastructure against cyber attacks. Big scale ones, such as those against "its railroads, its pipelines, its electric power grid, its aviation system, or its banking system."
According to Clarke, there are 20 to 30 nations that could launch a major cyber attack against the US and that such a concerted cyber attack is no longer in the realms of science fiction.
As many of you know, there has been a lot of hubbub about WikiLeaks and that many hackers, together acting as Anonymous, have brought down major corporations such as MasterCard and PayPal. Clarke, and others, use this an example of how large-scale cyber attacks could happen.
If you head over here, you can read some suggestions on how some people think the situation can be improved, including improved education and spurring new research into changing how software and networks operate.