This isn't as outlandish at it seems--modern China is smart, and can see that the Soviet Union is gone. China has established diplomatic relations with both South Korea and the U.S.. Idealism is being largely replaced by commercialism, and China needs allies in the west far more than it needs anything North Korea could offer.
If they offer their help now, when the western world isn't yet demanding it, they not only avoid the bullying Pakistan endured over Afghanistan, but they've got a much bigger bargaining chip than they would have as a forced late entrant. If there's a war with North Korea, China boosts its international image by being on the right side for once, and earns an important if not immediately influential ally in a reunited Korea.
Imagine what would happen if China becomes proactive--if instead of, say, America and its allies England and China going to war against North Korea, it's China and its allies America and Japan going to war. China provides the troops to take the heat off Bush, Abe gets to show he's serious about improving Chinese and South Korean ties, and suddenly two of the most important countries in the world owe China some pretty large favors. The communists get a huge boost in international relationships and something for the citizens to be proud of.
The cold war is over, but even the appearance of being in bed with the wrong people can have serious repercussions. In 2006 defending communism isn't anywhere near as important as attacking despotism.