Southern Methodist University in Dallas operates two nuclear blast detectors like this one–one south of Reno, Nevada, and another in Big Bend, in West Texas. No word from the university whether their instruments detected anything from North Korea, but since they’re funded by the Defense Deparment, they presumably are keeping mum until the Bush Administration issues its definitive report. Popular Mechanics describes the international detection system in this link from Instapundit.
Meanwhile, Pajamas Media thinks there’s little chance it was a hoax: "The emerging consensus is that such a trick would be very difficult; getting all of the TNT to explode in the same nanosecond is nearly impossible."
UPDATE Comes the confirmation. "The United States reported Monday that radiation-detecting aircraft had confirmed that North Korea conducted an underground nuclear test a week ago." Size: less than a kiloton. Which is impressive enough: even low-yield nuclear weapons are very powerful and destructive. "The nuclear weapon that destroyed Hiroshima had a yield of 10-15 kilotons, so weapons of up to five kilotons are still quite large." Call it a mini-Hiroshima. Suburb sized.
THE TEXAS SCRIBBLER 
Not necessarily a hoax, but a badly engineered device will “burn” a relatively small part of the plutonium/uranium and thus the explosion may be less impressive.
Wonder how much damage even a badly engineered nuke could produce.