Actually I tend to see scenarios like this less in terms of terrorists somehow getting a big nuke but in terms of North Korea finally snapping.
Man, that's brilliant! After years of everyone in Seoul, Tokyo and Washington having collective nightmares of a big-haired lunatic in platform shoes Kim goes and attacks China? I have to say it fits his MO perfectly - completely insane, absolutely unexpected and explcable only from an absolute conviction of superiority.
I actually think that the net outcome would be an increase in world happiness, though. Chinese occupation in NK means they finally get enough to eat and are free-er, the world can stop worrying about Kim's nuke and missile show, and people had catharsis and closure. Unless you were unlucky enough to be cast as a player in the catharsis bit, obviously.