People underestimate Halo 3...
You know, the game sold over 1.7 million in 2009 alone? It'll manage another million in 2010, I can't see it topping off anywhere less than 13mil lifetime. Even Gears of War 1, a much smaller game, is still selling bits, making over 700k since Gears 2 came out.
For MW2 to pass Halo 3 before 2011, it'll need to sell 3mil next year. That's asking for MW1-type legs, which I don't see happening. If the game ever does pass Halo 3, it will not be any time soon (definitely not 2010). Honestly, I don't think it'll happen at all (due to the oversaturation of CoD titles), though it'll be close.
edit: Really just depends on MW2's legs, or how it holds up next week, or next month. If Modern Warfare 2 ends up like Gears 2 (recall Gears had legs like MW1), then its sales will plummet after the holidays, and struggle to find new buyers, because all the owners of the first game already bought the sequel. We'll have to wait and see.
