The Ministry of Land, Infrastructure, Transport and Toursim announced the public notice of the land prices in Japan as of January 1st, 2010. Yes you now have an English page!
The trend shows a significant downtrend in all types of land. But interesting to see some sites in Hokkaido in the list of cities with land prices on the rise. On the other hand, in the list of sites showing the largest decline – the site lists areas in central Tokyo (i.e. Shibuya-ku, Minato-ku).
http://tochi.mlit.go.jp/english/index.html

General Overview Report
http://tochi.mlit.go.jp/english/land/01-02_21k.pdf

Land General Information System (to get an idea of land prices where you live)
http://www.land.mlit.go.jp/webland_english/top.html

Land Price Trends
http://tochi.mlit.go.jp/english/01_04.html