So North Korea has officially joined the nuclear club, by setting off a nuclear bomb in an underground mineshaft. It's not a good thing that a dictator like Kim Jong-Il has nuclear weapons.
This development, while alarming, was not exactly surprising. It's been suspected for a while that the DPRK has nuclear weapons.
However, there are a couple of signs that we shouldn't panic just yet. Firstly, we have no idea whether the test was successful. It seems that the explosion was smaller than that of the Hiroshima bomb (which seems to the standard by which nuclear weapons are measured. That's not particulary successful. Secondly, the North is still years away from being able to put a bomb on a missile. Thirdly, their missiles aren't the most effective things.
What we do have to worry about is North Korea's conventional weaponry- most particularly the capacity of their artillery to devastate Seoul. Sadly, this makes a liberation of North Korea a non-starter.
A setback for world peace, yes. A rephrensible act, yes. A sign of imminent Armageddon, no.