According to the video below, they will sell the game for $1 if 10 million people preorder for that price. If they fail to reach that target, all the money will go to charity and anyone who decides to still get the game will get a $1 discount.
There is literally no way in hell they will reach that goal, this is a marketing stunt (and a bad one) and nothing more.
Modern Warfare 2 sold 4.7 million copies on it's first day. Not all of those were preorders. Probably far less than half. How can they expect to get 5x the amount of preorders than one of the biggest games ever? Ridiculous.