The content pack can be bought on Xbox Live Marketplace for 1200 points, and for your money you get the next instalment in the game's co-op campaign, new playable maps, plus new games types and more customisation options. Chapter two doesn't appear to have made it onto Xbox Live just yet, but it should be arriving very shortly.
15$ is alot for 4 new maps and 4 new co op maps!! I guess you can play the 8 existing maps at different times of the day and it changes the gameplay alot. I will think a little more about this but will probably download them when I get home!
But the market place was full! Everyone was trying to download it and wasn't moving at all. I left it on over night and it finished.