I would not write off Halo 2 as a 10M seller too quickly. I can see Microsoft selling a Halo trilogy bundle with all 3 titles for $90 or so. If it does it during xmas 2009 it can get another 1M-2M out of the bundle alone.
Prediction made on 11/1/2008:
Q4 2008: 27M xbox LTD, 20M PS3 LTD . 2009 sales: 11M xbox, 9M PS3