UPDATE: I should have chosen my words more carefully. I noticed that news agencies were all reporting that North Korea claimed to have detonated a nuke. Then I saw a report of seismic activity from South Korea, which seemed to confirm the claim. But the size of the blast is now under scrutiny.
UPDATE 2: It looks like the size of the explosion was inconsistent with a successful nuclear test, and more likely either an unsuccessful failure or a deliberate fake explosion. From TPM:
Jane's Defense Weekly, a premier source on this sort of thing, says that if the initial reports of a .55 kT (half a kiloton) blast are correct "it would suggest that the test had been a "pre- or post-detonation" event (ie a failure), as it had been anticipated that North Korea's first nuclear test would have a significantly higher yield."Mr Snowjob says it could take a few days for the US to confirm whether it was a nuke or not, but mysteriously declined to explain how such a conclusion would be reached (hint: he probably doesn't know). The answer, it seems, is detection of radionuclide data.
Regardless, the test itself is a damning indictment of Bush's tough talk (no walk) policies, as Josh Marshall makes very clear here.