Since many of these issues have been on-going, one might ask, "Why now?" There are a couple possible explanations. First, Japan is in transition. The country, boasting two of the key pillars of world power (money and information) is seeking that third pillar--military force. Of course, this transition will require time. But the popular Koizumi government is in a position to initiate this transformation of Japan.
At the same time, China is awakening. As the country takes on a more vital economic role, it will inevitably seek to augment its economic clout with greater force projection capabilities in the region. Unfortunately for the Land of the Rising Sun, this vision of China-as-superpower clashes with dreams of Japanese military resurgence. In past struggles for position, Japan has always been able to play its economic trump card--its investment capital. But with China's rising economic strength, China can now make its own threats, as when Chinese protesters recently started calling for boycotts of Japanese goods.
South Korea's relationship with Japan is also troubled. Korean school-kids have anti-Japanese thought infused into their young brains from the first day of school. However, my guess is that the recent tirades against Japan are largely due to recent trends in domestic politics. Koizumi has, from time to time, tried to appeal to the more nationalist fringe of Japanese politics through symbolic gestures (visits to military-related shrines) or through tolerance of aggressive statements. Korea's President Roh, on the other hand, has grasped onto the anti-Japan crusade like a drowning man, trying his best to divert public attention away from his unpopular domestic record.
Of course, the long-term effect of all this exaggerated rhetoric is a great deal of animosity and confusion among the more hot-headed and less-informed sectors of the public. Those living outside of Asia should look at the current situation as a sign of things to come. China is waking from its slumber. It will demand a bigger voice in all things. Ten or fifteen years from now, it won't be happy to sit back while some American president determines the game and its rules.
P.S. Blog Adrian Wong also has a post on this subject.