A highlight of my trip to Japan earlier this year was taking the bullet train from Tokyo to Osaka and back. On the way down, I had plenty of work to do, and an empty train to do it in, but no way of getting access as we flashed through the countryside and cities. That may change, but it also will take a while.
According to a story in wireless.it.world, the Shinkansen will get Wi-Fi when its rolling stock are upgraded by Central Japan Railway Co. in 2009 to N700 series trains. According to details of the plan, the train will pick up digital network access from a coaxial cable running alongside the track, and redistribute the signal via equipment in each car. The system will also apparently enable voice communication from driver to guards, and enable multi-party discussions with train operators as well.
Meanwhile, it's all mobile-based access. And don't dare talk loudly on your mobile!