Ok, so this is for only one week but it's still the first time the Xbox 360 has outsold the PS3 and Wii in their home market.   I'd bet this is the first time ever an American game console has outsold a Japanese home console in Japan.  The reason for this spring past the Wii and the PS3 is the release of Star Ocean 4: Last Hope exclusively on the Xbox 360.  The title was on top of the sales charts selling a whopping 162,000 copies in it's first week.  The Xbox 360 managed to sell a little over 25,000 units vs 16,900 and 16,000 for the Wii and PS3.  The 360 was outsold by the DSi and the PSP which just goes to show Japan's movement away from home consoles towards hand held gaming devices.

Source:
Hardware Chart
Software Chart