Honest Abe's declaration in Peking that:
"We agreed that a nuclear weapon test by North Korea cannot be tolerated. We need to prevent a nuclear North Korea."appears to be a green light to Chairman Hu Jintao to give it his best shot. It may also have been a notice to China that "if you don't off this nutter, we will". It's not as if Japan doesn't remember how to get to Korea.
UPDATE: Upon reflection, there is a good possibility that the ChiCom troops have been moved to the Yalu to shoot refugees from North Korea in the event that China suspends all oil deliveries to North Korea. The cheapest way for China to force submission is to cut off the oil. As the North Koreans begin freezing it would be natural for them to try and escape. China really doesn't have a need for more peasants.