According to South Korean media, via AP, North Korea has detonated a nuclear device underground causing a seismic shockwave. They tried something like this a few years ago and it was deemed a "failure" by the world. This left a bit stronger seismic signature (4.7 instead of the previous 4.2) but that could simply mean they're testing kiloton warheads (hiroshima size) instead of a megaton warhead (the nukes that everyone else has). The story is here:http://www.google.com/hostednews/ap/article/ALeqM5iURO8fOyWVOA0ytFlaAGuC9F7R9wD98D0E580
My thoughts on the ensuing geo-political quagmire that may come about:
South Korea is going to react very badly to this because this opens the opportunity for the North to fire a nuke into Seoul (granted, they already have enough artillery aimed at Seoul that they can level it in less than 24 hours). Japan, who is already tempted to kick NK's ass for that whole rocket launch, will see this as a massive threat to national safety (I can only imagine what the news in Tokyo is like now, keep me posted Christa. I'm curious to see how Japan is handling it) and they may very well take military action against NK.
China may or may not support North Korea depending on the whether the west (which, funny enough, includes SK and Japan in it) fires first, which will make China intervene on the side of North Korea because they will feel threatened by aggression of the US and Japan near their borders. Or, by a twist of fate, they could see this as a horrible mistake on the part of the Koreans and side with us for nuclear proliferation against them.
Russia may join in on the side of SK and Japan in all this if China gets involved, since they feel threatened by the Chinese aggression along their border. Even if China sides itself with the UN/SK/Japan/US, the Russians may very well side with western powers because they don't want to deal with the massive backlash from the EU and US.
North Korea see this detonation (especially if it really was considered a success) as a bargaining chip because now they have something they can sell on the black market for food and medical supplies they need. Or they can say "look, we're not p*ssy teenagers anymore, we have the bomb. Listen to us." They're still acting like p*ssy children that are upset no one is taking them seriously, but thats besides the point. The North Koreans also pose a real military threat to the region. They have one of the largest armies in the world (I want to say 4th), and one of the largest navies in the world. The only problem, is that they kinda screwed their people over on the whole food thing.
The US, in all of this, will have to take a defusing role (forgive the pun). They will have to take a somewhat neutral stance in all of this. Something along the lines of "We don't want to start anything, but be warned that if you start something, we will have an army on your doorstep so powerful that you shall tremble as we march on your doorstep" Granted, they still outnumber us, but we have far better tools to do the job.