Talking to CVG, Lombardi reconfirmed that Valve still sees Left 4 Dead for PC and 360 only; however, he did leave some glint of hope for a PS3 port in the future. When asked if EA would bring the game over much like what the company did with The Orange Box, Lombardi replied "Left 4 Dead doesn't have that guaranteed appeal yet - it's a new IP. If you mention a new Half-Life, people want to make as many versions of it as possible. If Left 4 Dead is big, then we may see a PS3 version later, or if and when we do a sequel, people may be more interested in that."
In short, it all comes down to financial risk for whoever wants to take on the challenge of porting. As far as Valve is concerned, they're not the ones who will do it; someone else will do it if they want to, that is. Hopefully, Left 4 Dead does well on the other platforms; we could always use a zombie shooter on the PS3.