Treyarch unveiled an in an interview that its upcoming Call of Duty 5 game will feature a co-op zombie Nazi mode that will be unlockable after the singleplayer campaign is finished:
The game mode will be a survival variant that features up to four players who must work together to survive continuing waves of Nazi zombies who have an awful craving for fresh, patriotic flesh.
This survival mode will apparently have some strategy elements woven into it too and, by wiping out swathes of these unliving national socialists players will be able to earn money that can be invested in various upgrades. Players will apparently be able to unlock and buy new weapons as they progress, as well as add new defences on to the base they are defending.
It's a bit weird to see zombies in Call of Duty, but as long as they remain an extra and don't show up in the singleplayer campaign I'm OK with it.