North Korea doesn't have the ability to completely destroy the U.S. Not even close. At best, they could launch a missile that could hit Alaska or Hawaii, and even though they tested a relatively weak nuclear bomb, it's unlikely that they have the technology to make the warhead small enough to mount on a missile, so they're limited to a conventional (or possibly chemical) attack.
The U.S., on the other hand, could turn South Korea into an island if it ever came to that.
North Korea, however, could destroy Japan and/or South Korea. The United States would be blamed for allowing that to happen. Violent retaliation against North Korea would harm diplomatic relations with communist China. It's a delicate situation.
It is true, however, that North Korea as of yet poses no real threat to American longevity.
I don't know much about the North Korean military, but even if they currently have a nuke (and didn't use their only one in that test a few years ago), I'm not sure they'd have any means of delivering it, even to Japan or South Korea. I don't know if they have heavy aircraft capable of carrying what is likely a primitive, and therefore very large, warhead.
However, Japan and SK do have cause to worry, as NK could almost certainly launch a large-scale conventional attack against either country. It would probably lead to devastating retaliation, but Kim Jong Il hasn't shown himself to be the most rational person ever, so that might not deter him.
It's harder to place spies into North Korea, so the whole country is a black box except for satellite imagery. Japan, South Korea, and the United States probably have very little idea of what's going on in those borders.
