Joystiq is reporting that while the Xbox 360 version will ship with local co-op only, but online co-op will be added to the game on September 28, the same day the game releases on PS3 and PC with online play as well.
Additionally, Crystal Dynamics has announced that from "October to December the Downloadable Lara Croft campaign continues with 5 [DLC] packs available across all formats. The first three are expansion packs featuring a combination of all new puzzle, exploration and combat experiences in the game and new areas to explore.
"The further two packs are playable character packs, offering gamers the chance to experience the entire adventure without Lara Croft and Totec and instead with some other well-known videogame characters..."
What a shame. I'm pretty sure I'll be done with it by September!