On top of what wmf said, with whatever government investment has or has not already been made, the U.S. has the fourth highest* IPv6 adoption by percentage of requests as of Dec 2013 [1]. That is double what it was just six months prior [id].

Moreover, I doubt the US has as pressing a need to stave of address space exhaustion as do developing countries like China with rapidly expanding internet uptake.

*biased by Akamai's usage rates from country to country, of course

[1] http://www.networkcomputing.com/networking/ipv6-adoption-her...?

Why? Japan spent a lot of effort on IPv6 and what did it get them?

Why? To get widespread ipv6 support, I'd suppose.

IPv6 adoption on its own is pretty meaningless.. I suspect it had more to do with economics than tech.

This is total guesswork.. But here goes.. Japan likley invested heavily in IPv6 because their economy is very tech driven, the risks of running out of IPv4 or even being viewed as "behind" were likely the driving factors IMO

Cheap gigabit internet for all?

I'm not sure IPv6 had anything to do with the cheap or gigabit... Maybe the "for all"... But even that's a stretch :)