The Nikkei have said that Amazon will starting selling their eReaders in Japan as soon as next month. This is not the first time this rumor has surfaced. In fact in February it was suggested that Amazon would be rolling out the Kindle in Japan in April.
Two weeks ago, there was an update for the Kindle for the Mac which for the first time included support for Japanese. Coupled with the fact that other competitors such as Kobo launching in Japan, it would make sense for Amazon to launch sooner rather than later. They wouldn’t want to give their competition too much of a head start
No source has been named and no specific details have been given.