We already know that one of the big goals of Windows Phone in 2011 is expansion: more languages, more markets. In fact, it was stated by Andy Lees that they want 88% of the languages covered this year. Japan is obviously a huge part of that market, so it is not too shocking that Microsoft plans to launch there this year.
In an interview with one of Japan's largest papers, Sankei, Jean-Philippe Courtois, president of Microsoft International said "Japan is next on the list" for support. The big hurdle if implementing the Kana/Kanji writing system, something that takes some time. Adding Japan to the Windows Phone roster will help tremendously to WP7 expansion and adoption.
Source: Sankei; via Nanapho.jp; Thanks, tezawaly