According to Konami Japan's statement after translation:
"Due to various reasons, the launch of the Japanese version be made only PlayStation 3. We have been looking forward to the launch in the Xbox360, to everyone of the persons concerned, and
sincerely apologize for the inconvenience"
Weird. As far as I know that the Xbox360 is selling well in Japan, why would they cancel it?
Info: The PS3 version will release in Japan