Japan already has over 200 nuclear weapons ready for final assembly, according Jane's I think.
With the latest upgrade, the Dong-Feng 41 should be able to hit all US targets by 2010 with a projected range of 12,000 km.
Ah yes indeed, we can count on the Japanese to apply the principles of demand flow manufacturing and material staging in Kanbans even to building nukes! Gotta love 'em!