Kim Jong Il is full of crap. If it were a nuclear blast, we would know it. Too many people have too much equipment for it to not be detected.
Yeah, really. Who does this guy think he is—Dan Rather?
It's possible (though unlikely) that this was a calibration test of a large mass of conventional explosives, in preparation for a nuke test. It is not possible that this was an actual nuclear explosion—even assuming our government would keep that secret, there are too many independent scientists out there looking for precisely such an event for it to have been covered up.