Speaking with VideoGamer.com, Infinity Ward explains that they did not include co-op in the story campaign of Modern Warfare 2 because they are "crafting a very specific experience" that they don't want to "ruin with co-op". However, they plan on taking moments of the game that will work will in co-op and will include them in the Special Forces mode, similar to how the latest Turok handled co-op.
The only catch is that the new game mode is limited to two players instead of four. According to Infinity Ward, anything more than that "becomes a clusterf***", and so two players is the "magic number". A similar defense was used when Too Human shifted its co-op from four players down to two, a choice that overall made a couple character classes in that game lacking. So will Infinity Ward's decision prove to be accurate or only anger gamers further? We'll have to wait until the game lands November 10th of this year.