Left 4 Dead 2's Cold Stream DLC is out in completion on Steam today, but the Xbox 360 version is being held up (probably at gun- or frying-pan-point). Valve is working with Microsoft on a release schedule, the Left 4 Dead development team tweeted today. They hope to have it out "soon."
When Cold Stream does finally drop an anchor in Xbox 360, it will cost 560 MS Points ($7). Cold Stream is free for PC and Mac via Steam.