It is said that "absence of evidence is not evidence of absence".  In other words, just because there hasn't been a MAJOR cyber-attack is not evidence that they could not perform one.  What if they were just holding back to prevent their real power from being shown?  What if the Koreans or whomever performed the last break-in just did it on a bigger scale next time, with more programmers using more computers?  The point is they did it already using antequated computers (according to the US gov't), so it's not a large leap that if full war broke out, that they weren't already poised to do something big.  There are reports that China is already in such a stance.  To believe that the US isn't vulnerable is just living in a fairy tale.