I'm not sure what to think of the whole North Korea thing.
Though nuclear weaponry is no good thing, in many ways, I can see that North Korea's test makes the world a little safer. If the US were to launch one of its beloved pre-emptive strikes on China's ally, North Korea, then that would be disastrous for world peace, and North Korea having a nuclear capability makes that a bit less likely. In the current climate, having seen what happened to an unarmed Iraq, you can't blame them for wanting to beef up their defences.
The other thing to take into account is that being able to produce a nuclear explosion is only a small part of the whole equation. You also need to have a missile to get it places, and the whole fusing procedure is no picnic either.
A declaration of war on the US is only rhetoric for internal consumption, so its not something to get too worked up about, especially as the US is effectively at war with them anyway, just without the bombs, and only diplomacy prevents them saying so. The idea of their being able to sell the weapons to anyone else is practically impossible in the world today.
There is a (IMO) small danger of an attack on South Korea, but all North Koreans must realise that would be suicide. I don't believe even China would sit back or defend North Korea after such an act - China also realises what side its bread is buttered in the modern world and is not going to allow a small state like NK take it down into a WW3 scenario. The president is portrayed as a bit crazy but that may or may not be spin given to us, and he would nee a lot of crazy people down the chain to go down that path.
So the whole situation isn't good, but then again, its not something I'm going to worry about. You may not see it this way, but I am more worried by the prospect of what trouble America will get us into than anything else at the moment.
We spent the 80's sweating it out in the belief that the USSR was run by unhinged drunks and may wake up any moment and decide to push the button, and as it turned out, they weren't so different from us. A couple of years ago everyone believed a nuclear war was about to break out between India and Pakistan, but that didn't either.
Stop panicking, start living !!