I don't think it will happen in the near future, and I don't think it is inevitable nor even likely. Just that it is possible. Culturally, Japan is still very different from us and to a large degree is still in denial about World War 2. Currently they vote similiarly to America on many international issues, but their logic is a bit doggy -- they aren't voting that way because they have independently decided we have the best policy, and they aren't voting that way because they feel friendship. In a lot of ways, they have the attitude "The US beat us in WW2, so they are stronger than we are; therefore we should obey them". The problem is that if the US becomes too stretched or has a crises, Japan can easily decide that we are no longer stronger and they can start bullying their neighbors again.
Their new model of warfare is economic, and while they might have problems in that area right now, I think they enjoy the warfare of business over the other kind.
Now they darn well know that we can't police the whole world, and that their continued survival in light of Chinese and N.Korean sabre rattling will require more effort and commitment from them militarily. A fact bourne out by their recents actions of expanding military roles and budgets, and even offering troops in the middle east. I beieve they are also working, or at least talking about, ammending their post war constitution to allow it.