"We are supporting multiplayer with The Last of Us," game director Neil Druckmann told Gaming Examiner. "However, we can say that it is not co-op within the main campaign."
Naughty Dog's last game, Uncharted 3, featured a number of multiplayer modes, both cooperative and competitive. The Last of Us could feature a cooperative mode that lives outside of the campaign, similar to horde modes featured in other games. However, would a competitive experience work? It might be interesting to see if The Last of Us' lack of regen health and low ammo supplies can make for a different online experience.