Left 4 Dead Blog has a "worlds collide" post (presumably written by independent George) highlighting a bunch of mash-ups in celebration of the pending release of The Passing DLC where Left 4 Dead and Left 4 Dead 2 will collide. They also announce that the release of The Passing will come to pass next week, as they say: "Left 4 Dead 2's The Passing is coming out next week and we will be updating the blog next week with more news on the release."