And he has a quote from someone in the WPF dev team:
It was decided not to support vertical layout for East Asian text in the initial release of WPF as part of setting a reasonable limit to the scope for that release. It is a recognized gap that is being considered for a future version.
That was in 2007, and its still not there as far as I can figure out. Notice that there's a RightToLeft property on controls, but no vertical formatting property.