That would be quite an impressive feat, given the titles it'd need to trample in order to become king of the hill. In its 21 months on sale, the original Modern Warfare has sold 7.7 million copies across all platforms in the US, according to NPD. For comparison, Wii Play, which has sold more than 11 million copies in the states according to NPD, leads the all-time best sellers list for games that were never bundled with a console or handheld. If you consider pack-ins, it'd have to beat Wii Sports' nearly 24 million units in the States. To do that, every PS3 and Xbox 360 owner in the US would need to buy a copy.
So, yes: It could be the best selling game of all time. Emphasis on "could."