I'm going to go against the crowd here and say that Valve will make it paid DLC on all platforms.
It wouldn't surprise me at all. L4D1 was a console game pretending to be a PC game, and so it did all the things to try to convince everyone else it was just a normal PC game. With L4D2 it came out of the closet, so to speak, and now there's no reason to hand out free content just to try to fool people into thinking it's still a PC game.
I hope I'm wrong, of course. But like I said, wouldn't be surprised...