You can take part in the Game for Good weekend on the 5th and 6th of December, Activision will donate money to the War Child charity for every player on COD:MW2 over that weekend. (up to a quarter of a million squids). As far as I can tell, this is for UK players only, so if you are in the UK and have COD:MW2 make sure you get online on the 5th and 6th of December!
You can read more about it here; http://www.game.co.uk/gameforgood/