Kotaku notes that the Japanese release of Halo 3 works in US/NTSC Xboxes. Usually the reverse (US releases working in overseas consoles) is true, but not the other way 'round. The only catch is Japanese language audio with apparently some grammatical problems, according to comments.

UPDATE: According this story at Xbox360Rally, Halo 3 sold 45,000 copies in Japan its first day, more than half of what Blue Dragon sold in its first week.