On the surface, the people of 4 big cities have taken to the streets to protest new history textbooks being used in Japan. At first glance, this isn't much of an issue really and it would be hard to fault the Chinese people for being upset if Japan is down-playing what was done to the Chinese during World War II.
But, dig a little deeper and you start to find that the cracks in Japan-China relations are far deeper than schoolbook renderings of Second World War atrocities. The Chinese are making a big push for a dominant regional, if not global, role. They are in dispute with the Japanese on a range of issues (Taiwan, UN Security Council membership, disputed islands in the East China Sea), threatening the Taiwanese, making new deals with India, Pakistan and Iran.
China's trying to win enough friends to ensure that it's economic growth can continue unfettered while also flexing its regional muscles to keep Japan and Taiwan in their respective boxes. However, the Japanese and Taiwanese are not keen on remaining in their boxes. The Japanese are seeking a bigger role for themselves and the Taiwanese are getting close to declaring their independence from China.
Relations between China and Japan have soured to the point that rumors that Japan might boycott the 2008 Olympics have begun to surface.
The People's Daily (Chinese government run paper) reports that Chinese-Japanese relations are "at a crossroads" according to a Chinese State councilor. What he means is that Japan is not being the pacifist punching bag that the Chinese have grown used to over the past few decades.
It's worth nothing that street protests, such as we have been seeing in China recently, don't take place there without tacit government approval and they may actually being used by the Chinese government to deflect attention from their own failures.