Assuming that they have a workable nuclear device, they would have to drive one into South Korea and ship one over here to the US to be able to carry out their threat. They can't even get a rocket to fire to a set location.
we have the patriot system and others like it(even a laser truck system) for shooting down ANYTHING thrown at us. It would be a huge waste and big mistake. China knows we would take N Korea out leaving fall out to close to their borders. They'll take him out first.
Who knows... But I dont think so. If they would, it would be the last thing they ever do. And I dont believe the Chinese will risk their economical success by taking side of a bunch of idiots. I guess they would rather get rid of the obligations to their "socialist brothers", but they dont know how to do so without "loosing their face". And the North Koreans know this pretty well, I think.
Hmm it is now year 2013 and we are all still alive and not deceased in which everyone were appointing to the fact that we were all suppose to be dead and gone. Anyways they are just all talking and no action for now that is anyways....