Oh, good move North Korea -- get the Japanese pissed off. However bad the Japanese economy may be, they've got 10,000 times the resources of NK -- Japan will not take this threat lying down. I lived in Japan for 2 years in the late 1980s. Japan's been "dormant" compared to its historical outlook toward the world -- but if it chooses to, it can wipe NK from the map.
Look for Japan to become a major developer of anti-missile technology and/or partner with the U.S. in perfecting it. And something I've been predicting for some time may come true soon -- that the Japanese constitution will be revised to allow for a more proactive military presence in the world.
Japan's current defense policy:
http://www.jda.go.jp/e/policy_.htm
Back during the Cold War I used to think that rearming Japan would be a good idea (I mean, we rearmed Germany, didn't we?). But over the years I've come to have one problem with it. Unlike Germany, Japan has never come completely clean about their World War II era record. I don't think the Japanese are inherently cruel or ihuman--in fact, in the "1870's" they adopted a humanitarian code for treating prisoners-of-war and in World War I they were indeed humane to their German prisoners. But if they rearm without facing their past with total honesty, will it reawaken the World War II-era ideology? No, it wasn't Communism, but it was a Very Bad Thing.
I'm all for Japan rearming if we can be sure they won't go rogue. But if we can't then I'm not so sure about it--especially since there are still groups in Japan espousing the old WWII ideology and that, as "people of color," they don't attract the ire of the "politically correct" crowd in the same way that German neo-Nazis do.