I don't know if anyone else noticed but nuketown zombies is set before the events on moon. You can tell this because it is sams voice that announces the perks like 2x points and insta kill also the eyes of the zombies are yellow not blue yellow for when sam is in control of the zombies and blue for when richtofen is in control of the zombies. Also when you die you see the rocket from moon heading towards nuketown.

I have a feeling this is going to be tied in with the zombies story line heavily.