It's not hard for Japan to upgrade their BB infrastructure since the country is so compact. It was only a few years ago that their BB systems weren't as good as the US but now they they're into it, they just love jumping for joy and owning American .
However, the country is small comparatively. If Japan was the size of America and had that sort of BB density, then that'd be a real accomplishment if you ask me ;p.
The cable companies, Accesscomm, Rogers, Cogeco, etc., are increasinf everyone to a standard 5Mbit line. (the upload is isp dependant). There is an option for 10Mbit on my cable ISP. There are also some 9Mbit residential ISPs.
Japan's broadband situation absolutely destroys that of America's. No matter how fast your internet connection is, I guarantee mine is about 100 times faster and costs less (and doesn't flake out nearly as much).