The Cold Stream DLC, while still not showing on Xbox Marketplace, is currently available for download from the in-game menu. At the moment, it's showing as Free, despite being advertised as 560 points previously. Some people have said that the download stops at 99% (I'm not done downloading yet). Even if it stops at 99, the content will be linked to your account and can be downloaded later (unless Microsoft completely remove the content and re-upload it).
Cold Stream contains one new campaign, and the remaining Left 4 Dead 1 campaigns converted to fit Left 4 Dead 2