Probably never gonna happen. Nukes are super bad Juju in Japan.
Let's just say that Japan is sitting on a large stockpile of weapons grade plutonium for a "next generation" Plutonium power plant they never quite get around to building... :|
Current estimates indicate Japan sits perpetually about 2-3 months out from effective aircraft and missile-deliverable devices.
With the nominal way Asian cultures are heavy with double speak and "saving face" etc. I would not be shocked to learn that the "reactor Plutonium" is being stored as spheres that suspciously resemble "pits", and the JSDF has some "experimental non-nuclear" bombs or rockets that have these odd hollow warheads that were designed to study spherical explosive lensing and implosion...
It bugs the hell out of me that Japan never came to grips with it's WWII Imperial past the way Germany did, and I think we missed an excelent opportunity during the occupation to do so, but backed off from it out of fear they'd all commit seppuku or whatever.
Oh well, water under the bridge.
They're a stable and loyal and democratic ally of ours, and I would not mind them having nukes any more than I do the UK.
Hell, it'll give China and the Norks something else to aim at besides us.