Today Major Nelson has announced via his blog that in celebration of GTA IV's DLC release next week, Xbox silver members will be able to play both the original and the expansion for free online. This is part of a larger Grand Theft Auto week that will include giveaways, a Game with Fame night with Hip Hop DJ and producer Statik Selektah, an Xbox All Nighter on Feb 21, and a "Game with Rockstar" multiplayer event.
According to the press release details will be available at http://www.xbox.com/en-US/lostanddamnedliveweek
but at the time of this post, that link was not yet live. In the meantime, you can read the first half of the press release for details on the promotion and the second half for the product pitch for the Lost and the Damned DLC.