I just found this article from September last year. Apart from insightful information into the government-imposed resale price maintenance system for books, newspapers and magazines, it also reports that in the first month of introduction of KU on Amazon Japan, there were much more subscribers than Amazon hoped for.
I guess that means that there is bigger ebook market than estimated and possible phones as main platform.
http://www.japantimes.co.jp/news/201.../#.WOhpEm996Uk