Valve has confirmed that the Cold Stream add-on pack (or DLC, if you will) has had its release on the Xbox 360 delayed.
Both PC and Mac players are already able to download the update for free. Xbox 360 players of Left 4 Dead 2 will have to pay 560 Microsoft Points or $7 to pick up the Cold Stream DLC pack once it is released.
As to the reason for the delay, Valve has only offered that they are working with Microsoft to coordinate the release of the DLC and they hope that it is released soon. It seems that Microsoft can take its time in getting things approved, ready, and posted for purchase/download.