At least it's selling better than Gears of War (34K first week, 66K total) and Halo 2 (33K first week, 54K total).
Best performing XBox 360 game so far for Japan was Blue Dragon (83K first week, 197K total).
Looks like Halo 3's total sales will be somewhere in between, I don't think it will have much effect for the long run regarding Japan.