If ANY game were to outsell MW2, it would HAVE to be a multiplatform game, without a doubt. Every ps3 fan thinks GoW3 will, but hasn't the number of MW2 copies bought go over the number of ps3's even sold? I don't know, someone would have to do some googling.. or BING!!11!!1!one
Think about it. Fight fire with fire. Make an amazing game available to every console, even Wii somehow, and it. just. sells. Also if the developer chooses to have a decent marketing campaign like the Halo franchise has had lately, you get some interest as well.
Mass Effect 2 will almost certainly NOT sell as much as MW2. ME2 is an RPG with shooter elements, and gamers these days don't care as much for any kind of RPGs, save Fallout, but that's just more or less Bethesda at its finest. Almost every bestselling game in the past 5 years have been some kind of shooter. And with Modern Warfare's success, statistically speaking of course, EVERYONE on the freaking planet will own a copy if all goes well.
Honestly though, I don't see any challengers for MW's title anytime soon.
But there is hope for me, if i could make a game. Halo's controls and soundtrack, Uncharted's/Crysis graphics, Call of Duty's sound effects and blockbuster moments, Mass Effect's story quality, Fallout's sheer scale, Oblivion's item management, and just Liam Neeson thrown somewhere in there for voice talent, THAT would be a game i would probably make a mess in my pants for.