It seems that North Korea has successfully tested a nuclear weapon. At this point, until the "sniffer" planes land and their screens are analyzed, the best proof of that in the media is a shallow, magnitude 4.2 earthquake in the same region where the NoKos said they had successfully tested a bomb. The earlier report was of a magnitude 3.5 quake, equivalent to a 550 ton explosion. This could very well have been a conventional explosion designed to be perceived as a nuclear test, but the later upgrading to 4.2 magnitude suggests a weapon in the 1 kiloton range. That's a lot of conventional high explosives to fool everyone else into believing that the North Koreans have successfully detonated a nuclear bomb. The "sniffer" planes are right now patrolling the air around North Korea, trying to pick up any fragments of nuclear material that may have escaped the test site, things not found normally in nature that are definitive proof of a nuclear explosion. Until then, we have the word of the Russians that there has been successful detonation of something in the "Little Boy" and "Fat Man" class of nuclear weapons. The data so far suggests a far smaller weapon than Little Boy, which was 15kT. It's possible the North Koreans are the first to fail their intial fission bomb test, but it's also possible they're successful, and in any event it's disturbing that they want to be seen as such.
Surprising? No. This particular science experiment was first performed over 60 years ago. Given the proper parts, I could probably assemble the most primitive kind of nuclear weapon: one slug of highly-enriched uranium (HEU) slammed into another slug, making a critical mass and a few microseconds later, a nuclear detonation. I don't know enough about the analysis of nuclear test fallout to know if they'll be able to tell whether it was a plutonium bomb or a HEU device, in some ways a plutonium bomb would be worse, since that implies the ability to build an "implosion" device, which is much more sophisitcated but still a 60 year-old achievement. It's difficult to get a pure fission bomb to yield more than 40 kilotons of explosive power, a test above that level would be much, much more worrisome since it would imply a "boosted fission" or thermonuclear bomb.
If history is any guide, the delivery system for a first-generation nuclear bomb is basically, a train. It weighs a couple of tons, barely fits into the bomb bay of a WW II-era B-29, and is a cantankerous piece of hardware. In other words, this is proof of the ability to make a science project explode, but a long way from mounting one of these bad boys on a missile. Many more tests will be necessary before a nuclear weapon can be miniaturized and hardened to take the stresses of a ballistic launch and reentry.
The next logical question is, "What happens now?" Your 24-hour cable channel of choice will provide you with what is happening, but some additional background may be helpful.
The North Koreans field one of the largest armies on earth, at least numerically, but in real terms the greatest impediment to any significant change in the governance of the country is this:
Call it the 'Leper's Defense'. Nobody wants responsibility for the care and feeding of 20 million starving, indoctrinated Communist skeletons, so this would be a war no outside power is interested in winning. By making his country so impoverished and decrepit, Kim Jong-Il has successfully made his country nearly invulnerable. There is very little the North Koreans as a country do well, besides export arms and counterfeit currency. South Korea is fully aware of the difficulties that prosperous West Germany had in absorbing its Eastern bretheren, a decade's worth of economic disruption from which the united Germany has yet to fully recover. Twenty million more peasants in China would likely be a rounding error, but China doesn't want a direct border with South Korea and most likely doesn't want to be seen as the "occupiers" of North Korea, where the children are weaned on nationalism and very little food. The Japanese are the boogeymen of some North Koreans in living memory, despite sixty years of good behavior since, and while they would likely be ready to help with humanitarian aid they wouldn't likely be welcomed in any capacity. Something about their atrocities in the 1930s and 1940s makes them a poor candidate.
As far as the US provoking regime change, North Korea is lousy tank country and the South Koreans have no interest in the US starting a war and imperiling their economy. Seoul is closer to the DMZ than Dulles Airport is to the Capitol Building, and North Korean artillery can shell Seoul and its prosperous suburbs at will, possibly with chemical agents. While the North Korean nukes aren't deliverable per se, they make rather nasty things to leave behind for your enemies to overrun when you tactically retreat. The country of North Korea is very mountainous, favoring the defender and not ideal for the combined-arms (air-infantry-armor-artillery) operations at which the US excels. Even if the US wanted to invade it would take weeks to assemble the equipment and supplies for a protracted campaign. We did the amphibious invasion at Inchon once, I doubt they'll fall for that one twice.
The only option that seems reasonable at this point is a complete land-sea-air embargo of North Korea until there is regime change -- except that we're back to the Leper Defense. Who wants to "win" the care and feeding of North Korea, with goodness-knows how many stay-behind communists operating out of concealed bases? Relatively speaking, Iraq is an easier proposition, both in the conquest and in the post-conquest phase, even with the Islamic insurgency and the incredible amounts of IED-makings stored around that country. The best hope in this situation is that there is a military junta with sympathies to nominally-communist China (as opposed to the insanely Stalinist-Communist dictatorship there now) can take control, and in exchange for dismatling of the nuclear program (and weapons exports) the rest of the world can provide the aid the North Korean people so desperately need, as well as the connection to the outside world they so severly lack. Whether this can happen is beyond knowing, at this point.
The other very interested observers at this point are the Iranians, who are 5-10 years behind the North Koreans (we hope) in assembling of nuclear fuel and nuclear weapons. How the world responds to North Korea will give Iran tactical information about how to proceed with their own nuclear program. A drastic and effective world response that results in the overthrow of the North Korean government and Kim Jong-Il swinging from a lamppost is unlikely to encourage Iran to further pursue their nuclear ambitions, though deterrence is unlikely short of the US incinerating the North Koreans with nuclear weapons. That MIGHT get their attention, depending on how much stock they put in the supernatural abilities of the Twelfth Imam.
On the plus side, a couple of things are now clear. "Engagement" and the "Sunshine Policy" of the South Koreans is now shown to be bankrupt. Why the world needed further proof that you can't buy off a dictator forever is a mystery but here's the proof, yet again. The lack of utility of international bodies like the UN and IAEA when engaging a regime that doesn't respond to shame, isolation and sharply-worded communiques is also now apparent. Russia and China's responses to the North Korean test may cause them to reconsider their support for Iran's nuclear program, and it's possible that the Europeans will also realize that engagement has its limits with governments who are willing to talk while their centrifuges spin, then demand when their weapons are complete.
In the near term, expect Japan to repeal Article 9 of its Constitution forbidding offensive war. Japan has a large nuclear industry and extensive rocket technology, they'll be nuclear-armed within a decade if the Kim Jong-Il regime persists. South Korea's civilian government is a relatively new thing, most of South Korea's history has been as a military dictatorship. It's unlikely that the South Korean military will tolerate anything it sees as a half-hearted response to North Korea's nuclear provocation. The sun has most likely abruptly gone down on the Sunshine policy of engagement with North Korea, and if it hasn't there is high likelihood of a military coup there. This development is unlikely, in my opinion, to result in a South Korean invasion of North Korea. South Korea is relatively rich and prosperous, and has little to gain in an invasion, though if they chose to do it I imagine the US would help.
1. UN does nothing of substance.
2. US siezes North Korean ships at sea under the Proliferation Security Initiative.
3. Barring regime change, another North Korean nuclear test within a year.
4. Outisde chance of a complete embargo until regime change.
5. The media and the DNC will declare this test the result of failed foreign policy by the Bush Administration, and Bill Clinton will say that everything was fine on his watch, and that he came closer to killing Kim Jong-Il than any President before or since.