Quote from CaptainObvious:
Kim Jong Un has to win the respect of his military leadership. While he's some kind of General himself, he's never really done anything but watch parades.
Jang Song-Taek and Kim Kyong-Hui are a couple of top level crazies whom he will need to persuade he's up to the game. There may be a power struggle after the old man finally kicks, not before. Also there's the hundredth anniversary of Kim Il-Sung's birth coming up in 2012 and they'll have to celebrate some kind of victory party for that, and what better way than to launch a nuke.
I would guess that right now there are some high level talks going on between China, Russia and the U.S. as to just how North Korea gets thrown under the bus, and who get's what in return. Could be some people in the North get "disappeared". If not, it's pretty much a given that North Korea launches a nuke in the not too distant future. They are not rational people, so thinking it doesn't make sense for them to do it really doesn't come into play.