We're at double-secret alert-level orange (at least until the polling numbers improve) and President Bush has unleashed Pentagon hackers on the world, but actual experts say cyber attacks on the U.S. would be hard to pull off and of dubious value.
Turns out it is just very difficult to overcome the built-in resilience of the Net which, after all, was intended to survive not just malicious code but throw-weights by the megaton. It might be possible—with a lot of work and luck—to, say, bring down part of an electricity grid for a few hours, not unlike a bad thunderstorm might.
However, that is the kind of thing that only has value in service of some larger objective. Like turning off the force field so you can attack the Death Star. In and of itself, however, a power-sapping stunt has limited utility.
Some Ewoks, however, might throw you a great party.