Steam downloaded Cold Stream for me - I started up L4D2 and it was already there. ?
Fun map, they definitely throw waves of zombies at you. The final 'rescue' section is a little reminiscent of The Parish in that you have to run a ways through waves of zombies to get to your rescue chopper.
Most obvious bug: the collapsing bridge will basically slo-mo everyone's game for a bit till all the chunks are done falling. Kinda induces panic if you're not expecting it, because the falling bridge is accompanied by a horde >.<