A leak has stated the game is in progress. No surprise. It will most likely get the full blown announcement treatment at E3 2011.

According to Kotaku:

The game opens moments after the cliffhanger ending of Modern Warfare 2 with the U.S. struggling to stave off a surprise Russian attack. The single-player campaign will ping-pong players around the world as they take on the multiple throwaway roles as a Russian Federal Protective Services agent, SAS Operative, tank gunner, and AC-130 gunner as well as key characters from previous installments and new recurring characters. The game will feature about 15 missions, kicking off with the invasion of Manhattan by a Russian force and wrapping up with a final encounter in Dubai.