According to recent entries in Destructoid, Infinity Ward, now known as Modern Warfare 2, it is currently seeking new staff to develop a MMO for the series Call of Duty.
Also according to these rumors, the study of development would be asking for help to developers of Sony Online Entertainment and Blizzard, and would take nothing less than the lead designer of EverQuest II, Ryan Barker.

After rumors of yesterday, who wanted the Games Sledgehammer made by Activision for the development of Modern Warfare 3, all this seems to have now more complex story lines, but since there is nothing official you should take it all with due precautions.

Source: SpazioGames.it