None. Everyone knows the game is coming out, its not a surprise. The reason the stock price is in the doldrums is that wallstreet wants more. For Activision's stock to increase they need to maintain the revenue coming from their base, and find new sources of revenue to make up for revenue winding down from WOW and also add revenue on top of that to show the company is growing.
I'm not an expert, and I don't have a position, but expectations are already very high - so those would have to be exceeded to bring a pop. I don't see it - my gamer friends are tired of COD - they still plan to buy the game, but I wonder if everyone who bought MW3 will.