That certainly accords with what I see: they are allies, with a strong treaty that neither has made any mention of revisiting that I have seen. There are delegations going back and forth; Kim Jong-Il himself went for a nice visit in April, and another delegation is sallying forth soon:
DPRK Delegation to Pay Official Goodwill Visit to China
Pyongyang, October 12 (KCNA) -- The delegation of the Democratic People's Republic of Korea led by Kim Yong Nam, president of the Presidium of the Supreme People's Assembly, will soon pay an official goodwill visit to the People's Republic of China (PRC) at the invitation of the Standing Committee of the National People's Congress and the State Council of the PRC.
It is my belief that you wouldn't see such going on if there were overt hostilities about to begin. Apparently, Prime Minister Koizumi from Japan is not welcome in the PRC merely because of his visits to Yasukuni. I would assume that if China were truly upset with its lackey state, it would also make it apparent to them via the cold shoulder.
For that matter, Kim was in Beijing just in April. If they wanted to terminate his leadership, they could have just taken him at that time. It certainly would have meant less effort than a full-scale assault across the border a few months later.
My guess is that Kim's satrapy is in no danger from China.
A goodwill visit? That sounds more like a cover for talks with China about how to avoid a military conflict. China has no problem engaging in bi-lateral talks with North Korea if it serves the interests of China. My guess would be that these meetings are essentially made up of someone from North Korea pleading with someone from China not to attack, and someone from China demanding they give up their nuclear weapons program. It looks to me as though China's dropping into some shotgun diplomacy.