9th June 2010
From a screenshot of Qore's Guardian of Light section, posted by a Tomb Raider fan at the TR forums. It has been revealed that the Guardian of Light might be released on July 27 2010.
So far there hasn't been a confirmation by Square Enix or Crystal Dynamics if this is true, but we will post the details when we know more.
Possibly the answer might be revealed at this year's E3, since we know that people at Crystal Dynamics will be there and release brand new information.
There is also new screenshots posted by the same member from the Qore site. They also may contain spoilers from the game, but there is a warning before that point.
Click here to view the forum thread at Tomb Raider Forums about the release date.
Click here to see the screenshots.
UPDATE: Eidos has confirmed this is not the correct date for the release, and no specific day has been determined.