I also went to a lecture hosted by Sig Hecker, a former head of the Los Alamos National Laboratory and a current nuclear proliferation monitor, who said that China is unlikely to support North Korea in an offensive military engagement because it (China) is too well-connected to the rest of the world. It's content to have North Korea as a sort of screwy buddy, but it's not going to save the country's ass unless it's in China's own interest, i.e. if the China's borders are threatened.
By the way, another thing I got from the lecture is that it's not really worth mentioning North Korea's nuclear program, although it's advanced substantially they still only have about two nukes and they have precious little fissile material. It's possible that North Korea would engage in unilateral action against South Korea, but it hasn't done that yet, which suggests it's aware of its chances.
So, you can look up anything Hecker's written, he's one of the few people who's been able to get inside the country to examine its facilities.