News@Nature has the best summary of what is known about North Korea's missile test that I have read thus far.

How big was the blast?

Estimates for the bomb's yield (the amount of energy discharged when the weapon is detonated, in the equivalent mass of trinitrotoluene, or TNT) differ widely, from 550 tons of TNT to 5-15 kilotons (this last a Russian estimate). By comparison, the Hiroshima bomb was about 12.5 kilotons.

The lower estimate is very small: it would be difficult to build a bomb with a critical mass of plutonium that creates a blast like this. It may be that the bomb didn't completely detonate, or that the chain reaction didn't go to completion.

Read the whole thing.

UPDATE: It would appear that early reports about a very small bomb or failure to detonate were accurate. From CNN:

The United States believes North Korea attempted to detonate a nuclear device and that "something went wrong," a U.S. government official told CNN Tuesday.

The official confirmed North Korea informed the Chinese government prior to the test that it would be a 4 kiloton nuclear device.

The official added that the unexpectedly small blast, of a half kiloton or less, indicated "something went wrong."

Nuclear tests in 1998 by India and Pakistan were about 24 to 50 times as powerful - or in the 15 kiloton range, according to the Federation of American Scientists.

The United States still cannot say for sure the underground detonation was a nuclear blast; the working assumption is that it was, but not very successful, the official said.

Another U.S. official said it is possible North Korea may attempt a second test, but cautioned there has been no evidence of any preparations at another test site.

"I would not say we expect it," the official said on condition of anonymity because he is not authorized to speak publicly, "but it would not be a total surprise."

Russia's Defense Minister said Tuesday that his agency has estimated the power of the explosion at between 5 and 15 kilotons.

But the U.S. intelligence community is sticking by an estimate that the blast was approximately a half kiloton, or even less, although it's possible the hardened tunnel in which the test took place could have "muffled" the seismic waves, an official told CNN.

Considering the failure of the Taepodong-2 missile test last month, it would appear that the North Koreans are not as good at producing weapons as they would like us to believe. This is not to suggest in the slightest that this is not something to be taken seriously. People who test nukes of any size need to be taken very, very seriously. However, it should probably figure into our strategic calculations whether or not they can actually make a nuclear weapon work.

Just for contrast, I was hearing one intelligence analyst saying that Japan -- if they decided to go nuclear -- could probably build one in a "long weekend." They have the plutonium, and they most certainly have the technical know-how. This is an another interesting contrast that I think we should factor into our strategic calculus, both for the proximate situation and in terms of our long-term goals.

North Korea, because they have a closed society with limited ability to innovate, is going to have a hell-of-a-time even getting an nuke to work. Japan, even without trying, could bring themselves to strategic parity very rapidly. Which one -- from the perception of their own citizens and strategic planners -- is the safer country? My argument would be Japan hands-down.

If North Korea thinks that nukes will make them safe, they are about to have a rude awakening. If the proof is in the pudding, it would appear that being in a free society makes you safer.