You do realize that the zombie mode was a Treyarch invention not an Infinity Ward invention. I doubt Infinity Ward would add such a thing in MW2 when it is something that Treyarch likes to do.
Anyway, I have only had the game for a couple of days and beat it on Veteran the first day in a little under six hours, finished over 70% of the Spec-Ops mission by myself on the second day, and now have a total 1000 in gamerscore after some clean up with a buddy on the third day. Meh, I'm done with the game now. Now to start getting into Left 4 Dead 2!