It's no secret Valve isn't too interested in developing for Sony's platform, hell they even handed over development of the PS3 version of Orange Box to EA's UK team to keep their hands free of it. Not only did it suffer from sub-par frame rates compared to it's Xbox 360 and PC counterparts, but it was also excluded from any of the DLC Valve released post release for Team Fortress 2, such as maps and most importantly patches to fix online multiplayer matches. The mothafucking hip-hop. Here's a direct quote from Valve's Tom Leonard, via Loot-Ninja.
"The PC and the 360 are just more straightforward. We can focus on what we want to do, which is make game experiences, instead of sweating bullets over obscure architectural decisions they make with their platform. [...] I didn't come into this business in the 90s because of some technical fetish. I came in because I wanted to give people experiences that made them have fun."
That's quite the subtle hint. So that's why you shouldn't be expecting Left 4Dead, let alone it's recently announced sequel on your PS3 anytime soon. Take it from me, at least on console, you aren't missing out on much. Unless you're really into zombies. If that's the case you can at least look forward to Dead Rising 2.