All Games Beta. A product listing on the UK retailer GAME listed multiplayer as a key feature in the new Tomb Raider, specifically stating that players will be able to "play a variety of multiplayer modes as Lara's Shipmates or Yamatai's Scavengers." The description has since been removed from the product page.
Square Enix recently teased a "special secret announcement" in a video released earlier this year, promising that it would "change the way you play Tomb Raider forever." Assuming GAME's info was accurate, a multiplayer mode would certainly be new territory for the Tomb Raider series. We just hope there isn't a "Lara vs. Exotic Animals" mode. The animals wouldn't stand a chance.
Update: Official Xbox Magazine's latest issue confirms the existence of multiplayer, though specifics are still in short supply. We'll keep our ear to the ground for official details as they surface.