Was it a dud?
Arms Control Wonk is speculating that the North Korean bomb test was a dud. Although there was a seismic event at the time and place claimed by the North Koreans, it was smaller than anyone's first bomb has ever been. Most tests are in the 20 kiloton range and the North Korean test looks to have been in the one half to one kiloton range. If correct, this would be good news, but it still doesn't offset that fact that they were confident enough in Bush's impotence to call his bluff in the first place.
Update: Kevin Drum also suspects a dud.