Put this one in your back pocket...it's thin. Several outlets are quoting Infinity Ward Community Manager Robert Bowling from yesterday's Call of Duty: Modern Warfare 2 event, and they're running wild with the possibilities. So far, the most plausible theories have been in regards to the single-player campaign story; Infinity Ward has a reputation for outdoing themselves with incredible setpieces, cutscenes, and all-around production value.
However, one of our favorite growing gaming blogs, Loot Ninja, has its own theory: co-op in the main campaign. While we're not necessarily in agreement with that prediction, we're not opposed to what's being suggested! With November 10th creeping up behind a slew of October games, we may not get the full details until we have Modern Warfare 2 in our hands...but just in case, we'll keep our ears open for rumor control info.
Source: Loot-ninja.com