I know I'm a bit late to the party here, but I'm not worried about the future of the Call of Duty series at this point. While Treyarch hasn't always been a great developer in my eyes, the promises and hopes for Black Ops are all very high, and if pulled off, we may have a new "king" in terms of Call of Duty developers. As for Infinity Ward, I can see them remaining, but, as with the rest of us, I'm not really sure as where I think they'll be a year from now. Either way, I'm still expecting to get some good titles from them.
More than anything else, though, I'm looking to see what the future holds with Respawn Studios. With two legendary leaders, Jason West and Vince Zampella, I do believe that Respawn Entertainment will make their mark on gaming just as Infinity Ward did initially with Call of Duty 4.