While I agree with my friend Anti's analysis of Kim Jong Il's intentions - which is badly influenced by a case of severe attention deficit disorder, imperial narcissism, empathic lacking, and bloodthirst - I must point out that the Japanese Constitution is not, in fact, "pacifist." If we go with a typical definition of pacifism - the belief that disputes between nations can and should be settled peacefully, demonstrated by refusal to participate in military actions - we see that it is not pacifist. Why? While the text of Article 9, the famed "No War" Clause, states that Japan renounces war and its use to settle international disputes, the interpretation of that article has long held the Japanese right to force in self-defense. This has allowed Japan to build and maintain strong armed forces, as well as participate in international conflicts.
As a result of this interpretation, Japan has some of the best (if not the best) defensive forces in the world. These forces, however, would not be able to successfully carry out a "preemptive" attack on North Korea. They are simply not set up or trained to follow through with such an action.
Essentially, Japan is doing what it can - making threats, empty or not - to protect itself and its citizenry. I certainly think it's foolish to expect that a targeted attack on North Korean nuclear facilities wouldn't lead to international bloodshed, particularly among South Korean and American soldiers amassed on the DMZ. But I don't know what a good plan would look like in this situation...which is probably why the Bush Administration has largely ignored it.