Think about it ... NK decides that it's finally time for reunification of the Korean peninsula - their way. They lob a missile at the west coast (Los Angeles hoping to trigger the faults for a twofer?) to keep us busy. By the time we can retaliate, their millions of soldiers will have all of Korea if nothing more than a search for food. So ... we nuke the bejesus out of NK? They don't care, it is already stone age. They are all in the South by then. Nuke SK? Oh, that'll help. And oh BTW, Japan might have a few concerns about being downrange fallout wise. China might be a little concerned about that too.
You don't have to do this poll for Japan; they are already signed onto our missile defense program -- in fact, for the first time since 1954, they are not going to budget for a new destroyer in the next budget, and will instead redirect those funds for missile defense. I believe that the figure that they have in mind is $10 billion or so dollars for the overall project over the next few years, though it has been a few weeks since I had time to follow this one. (I did glimpse an article somewhere that indicated that there may even be more money allocated to defense that had been originally planned -- interesting, since I had seen earlier indications that there actually might be a small decrease.)
In my opinion, North Korea is becoming perilously close to finding out how close to the truth the FAS's estimation of Japan as a paranuclear state is; even more interesting is Global Security's very similiar recap of recent events in Japan.
I personally think North Korea is poking at the wrong porcupine here. While they claim not to have forgotten the lessons of World War II, I think North Korea is badly mistaken if they think that the spirit of the Kempeitai is entirely eradicated -- Kim's little program of disruptors/chosensoren might well prove to be shot through with double agents and intercepted information.