It appears that the Japanese edition of the Sony PRS-T1 reader does have a Japanese dictionary, and the ability to display text vertically. IIRC, even before epub3, the XMDF format allowed the Sony Reader to do this. (I don't think the US version has this ability though)
See this page for a sample of epub 3 using the Sony Reader.
(this blog also has some info on the Kobo reader and Japanese)