Crystal Dynamics has confirmed that Lara Croft and the Guardian of Light will be released on PlayStation Network and PC via Steam on September 28th for $14.99. While the release of the PS3 and PC version was to include the online co-op, it looks as though it will be released at a later date, presumably at the same time as it will be available for the XBLA version.
In addition, they have announced that from October to December the Downloadable Lara Croft campaign will have 5 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.