The combat walker Dreadnought may be stomping its way into Warhammer 40K: Space Marine in January. Relic Entertainment teased an intriguing image (above). Though the studio didn't use the word "Dreadnought" explicitly, that seems to be the reasonable conclusion from a healthy chunk of the fan community.
The tweet (via Game Infomer) says a "big metal surprise" is coming in January. Users of the official forums quickly agreed that it must be the powerful Dreadnought, and looking at an image from the 40K Wiki we're inclined to agree. The teaser image seems like a dead ringer for the mech suit.
The exact nature of the content hasn't been announced, so the notion of Dreadnought DLC is still in rumor territory. We'll probably hear more details of the big metal surprise soon, since January is just around the corner.