Cult Of Rapture has some details on the forthcoming extras:
The Rapture Metro Pack is coming soon for Xbox 360, PS3, and Games for Windows Live for 800 MS Point or $9.99. The pack includes six brand new maps including areas rebuilt from the single-player game for MP modes. We also have three new achievements and trophies and have added the Rebirth Feature for players who have reached Rank 50 - using Rebirth, you can rank your character down to Rank 1 and receive a special mask.
Still no release date, however. (I have to admit I blasted through the single player, but didn't bother to touch the multiplayer. Is it worth taking a look at?)