Look, put aside all these denials from Valve, saying they're not doing a PS3 version of Left 4 Dead. Of course they're not. They didn't do a PS3 Orange Box (EA did), they don't have in-house PlayStation developers, it's just not something they're interested in. But EA? Man, EA are interested in the idea of a PS3 version of Left 4 Dead, presumably because it stands a chance of making them some money. Gabe Newell:

For The Orange Box, EA did the PS3 version. They're interested in getting a PS3 version done. We just have to figure out where the resources can come from. We don't have the bandwidth. We're doing the 360 and PC version ourselves.

Sure, EA are interested, but if they can't do a better job on the port than they did with The Orange Box, we won't be. Gabe Newell Talks about Valve's Plans [1UP]