The the Rapture Metro DLC Pack now available on Xbox LIVE Marketplace for 800 Microsoft Points and from PSN for £7.99. It's not yet available for Windows - this happens at 'a later date'.
There are six "new" maps take from "areas from the single player portion of BioShock 2". Yes, single-player 'portion'.
2K also announced "two separate BioShock 2 multiplayer expansions:
Kill’em Kindly is free and sees players fighting to the death using golf clubs only.
There are also two new characters - Zigo the Fisherman and Blanche the Actress
- for the multiplayer at a cost of 160 Microsoft Points or £1.59.
Here's the official list:
• Six new maps
• Rank increase to level 50
• Rebirth feature that will give players who have reached Rank 50 the option to rank down to level 1 and receive a special mask
• Three new achievements/trophies.