In his Washington Post column today, Charles Krauthammer joins the list of those advocating that the U.S. let Japan go nuclear. To me the best argument for this approach is that it is the only surefire way to pressure China into using its leverage against North Korea. We don't have to actively help Japan obtain nuclear weapons, we can just engage in the same diplomatic doublespeak that China does with regard to North Korea. The State Department can issue statements "strongly discouraging" Japan from seeking nukes, but then stymie any efforts to impose sanctions on them through the U.N. We already have a history of accepting new nuclear states when those countries are our allies and it is in our strategic interests (Israel and India come to mind as prominent examples). We may not even need Japan to actually go nuclear, as long as it looks realistic enough that they are going nuclear to twist China's arm.
The main issue I have with the "nuclear Japan" argument is that it's unlikely that Japan, the most anti-nuclear country in the world for obvious reasons, would aggressively pursue nuclear weapons. Duncan Currie takes a look at this issue over at the Weekly Standard. I don't think it would be diplomatically advisable for us to try and actively convince Japan to acquire nuclear weapons.