Frankly, I think any celebration of the resumption of the six-party talks is premature, as Chris Hill, assistant secretary of state for East Asia and the Pacific, admitted: "We are a long way from our goal still. I have not broken out the cigars and champagne quite yet."
All that North Korea's return demonstrates is that China may still have some influence in Pyongyang. The scenario is easy enough to imagine: "Look, we're working to slow the sanctions process down and keep this thing from getting out of hand, but you have to give us something in return. Just come back to the talks. It will preserve the illusion that more talk might lead to some positive outcome." Expect the next round of talks to be more useful in communicating the conditions of the new containment regime than in moving closer to disarmament. The US and its partners should make it crystal clear to Pyongyang the consequences for any misbehavior with his nuclear weapons and material. Thereafter, the crisis will continue as a kind of uneasy stalemate, maybe with an incident or two at sea that pushes the situation to the brink of war.
Daniel Drezner, meanwhile, points to reports that China actually cut off sales of crude oil to North Korean in September, and speculates why China has underplayed the potential success of its backdoor sanctions.
I disagree with Dan that China's actions involve fears of a unified Korea. After all, what does China have to fear from Korea?
The real question is how a unified Korea would affect Japan. In Japanese strategic thinking, Korea has long been viewed as a "dagger pointed at the heart of Japan." A unified Korea, with or without nuclear weapons, would make Japan feel uneasy, and would likely accelerate the remilitarization of Japan, which China has no interest in seeing happen.
So Kim is useful only insofar as he keeps the US and Japan unsettled without pushing them to drastic measures. But should Japan commit to a major remilitarization program -- still unlikely for the time being -- Kim's usefulness to China is at an end.