When Lara Croft and the Guardian of Light launches on Xbox Live on August 18th, it'll be a strictly offline experience.In standard code for "It's not up to par", Crystal Dynamics global brand director Karl Stewart has stated that, "We made a decision recently because we were focusing on how we can deliver the best possible experience to the players. If we had launched with online co-op, we wouldn't feel comfortable that it was the best possible experience for the player."
Apparently the online multiplayer will come along later, in patch form.
Source:
Joystiq