Hmm, hard to say. It's trending slightly below Halo Reach in terms of preorders in North America. Its EMEAA performance is probably going to be the determining factor. I'd say it'll just edge out 6 million by the end of the year.
:edit: Although there is one other thing to consider. Last year at least some of Reach's success came from 360 flying high from the redesign and Kinect. Most indications suggest that there won't be anything like that this year so 360's overall sales might impact this games holiday performance. Although I still think it will beat 6 million, but it'll probably be really close.