Given the most recent Japanese sales figures (360 sold 30,000 units in a single week), one might expect Final Fantasy XIII to appear on the Xbox 360 in Japan as well, however head of Xbox Japan, Takashi Sensui says that there are no plans for this.

“We have similar high hopes for the upcoming launch of other RPGs such as The Last Remnant, Star Ocean 4: The Last Hope and Fable II. However, we have no plans to launch Final Fantasy XIII in Japan.” - Takashi Sensui

Source: FF-XIII