To make sure everybody sees tis I am posting it in the Beijing and Shanghai forums.
It is now possible to buy train tickets online for the fast trains.
The website is: http://www.12306.cn/mormhweb/
The tickets bear a name so you need to provide your id which , in the case of a tourist, is your passport number. You enter the information when creating the profile.
You later pick up the tickets at the train station (which you can do just before departure) but showing the email confirmation that you received or your file number. The confirmation can also be sent to your phone.
For next week, the cheapest ticket from Nanjing to Beijing is 455RMB compared to the cheapest being 550RMB for flying.
This, as I predicted, is nearly the same as buying plane tickets online.
The only problem is, for now the website in only in chinese. An english website should be available eventually. Ctrip.com has said that they plan to sell train tickets on their website. No idea when.