I'm going to go out on a limb and be the first to say that North Korea indeed tested a nuclear weapon. Using non-classified imagery I will explain to you what I think happened. North Korea wanted to test a nuclear weapon clandestinely. They have almost no experience in doing so and so likely collected information based upon previous tests of the US, Russia and China that were readily available. I believe they conducted an underground test of a 10 kiloton (give or take a few kilotons)weapon.
Click on image for full size
In 1970, the US tested a 10 kiloton weapon in Yucca Flats. The device was buried 900 feet down in a shaft, but it was not deep enough. A cloud of radioactive dust and debris was sent upwards--a cloud almost identical to that of the one visible from satellite imagery in the North Korean incident. Several of the people nearby observing were poisoned by the radioactive dust and debris.
North Korea looks like they have experienced exactly the same thing. This means anyone in that immediate vicinity could likely have received a fatal dose of radiation. The behavior of the North Koreans, including allowing unprecedented inspections --and then being led to a false site is already enough to know something happened. They are just awaiting the release of this. I guess I'll be the first to declare publicly what most good analysts probably already know--North Korea tested a nuke underground. They botched the test and it released a small cloud of radioactive debris. North Korea is now a nuclear power.
Just lifting the curtain, yet again...
UPDATE: South Korea has their own explanation
This is getting pretty funny. South Korea says it was a natural seismic event and the cloud was just a natural formation. North Korea says it was not natural but part of a hydro-electric project.
I want to remind readers that we did not detect the three nuclear tests conducted by India in 1998. They announced it to the world and later it was determined nothing unusual could be identified. No seismic "signatures" were identified. No radiation was identified. The whole world simply had to take their word for it. North Korea will not announce their detonations. They will lie and cover-up any such detonations. The weak explanation offered indicates that is exactly what we are observing.
Ambassadors, who had been led to a site 60km away from the actual seismic epicenter, were told by NK that more explosions would be forthcoming. Obviously there are many mountains that need demolition work. Or perhaps it is just more "natural" seismic activity and the North Koreans are only imagining they are building a hydro-electric project to increase their imagined prestige? Any weird mushroom clouds observed at these same times will just be coincidental natural weather formations.
Why cover it up? Amazingly, at this time no one really benefits by catching NK testing nukes. Seoul is enriching their own uranium and obviously preparing for the worst and don't want to bring more attention to their own nuclear activities. That is why they are so quick to give "natural" explanations that conflict even with North Korea's own contrived explanations.
Why lie? Because the US has indicated it might soon remove many of our forces which have recently been viewed unfavorably by a left-leaning populace. They clearly want to buy some time to catch up. By not pressing the issue and making their own excuse for it, they hope to keep Kim Jong il's government pacified and unprovoked. They realize the US isn't going to be able to destroy NK's nuclear weaponry anyway--it's already too late for that. Our troops would not be of much use if North Korea uses nuclear weapons on Seoul. Already their presence is superfluous. The leadership obviously realizes that a nuclear détente is now the only true defense against a nuclear North Korea--and what's more, I'd be inclined to agree with them.
So this will be explained away and covered up by both the west and the east--most likely until such a time as South Korea has nuclear weapons of its own.
Welcome to the bizarre world of dealing with threats to one's national security.