Valve Software, the studio behind the Half-Life series, The Orange Box, and the upcoming Left 4 Dead talked about why there is no PS3 port of Left 4 Dead currently planned.
Valve's Doug Lombardi told CVG, "We're not PS3 developers - we're doing PC and 360 like with Orange Box. EA came to us and said 'Wow, Orange box was an incredible project, can we do a PS3 version?"
Remember that the PS3 port of The Orange Box was handled by EA and not Valve.
This could also mean that EA might want to make a PS3 version of Left 4 Dead. If you don't remember anything about the upcoming zombie survival co-op FPS, take a look at this: