Lara Croft and the Guardian of Light is still scheduled to release on Steam and the PlayStation Network on September 28th - that's a week from today. But like the Xbox Live Arcade version of the game, which was released in August, these versions are going to be shipping sans the online co-op mode.
Apparently the team at Crystal Dynamics is having a bit of trouble getting the mode to work properly. Originally the plan was to have the online co-op mode patched in on the 28th for Xbox Live owners - but now it appears all versions of the game are going into a "wait and see" approach for online co-op. We were told it's definitely still coming to the game, they just don't have a firm grasp on the date quite yet.
It looks like you're going to have to keep enticing your local friends with free pizza and beer to get them to play some co-op with you for the time being. We rated Guardian of Light quite highly and were really only disappointed with its lack of online play.
The game is still scheduled to receive 5 DLC packs over the next few months, with the first one being free for Xbox 360 owners of the game for 30 days. It's unclear if the current decision to delay online co-op further will alter the deal - or have it carry over to the other platforms as well. We'll update you as soon as Crystal Dynamics releases more info.