Well, I can see where this is going. I'm just going to drop my official opinion and then leave, and never come back.
Valve is a company. They may seem like a "For the gamer" kinda thing, but at the end of the day, the employee's have to sell games to feed their families. Therefore, whatever route is profitable enough for them, they shall take it. Games take months, even years, to make and test, not to mention Valve is currently swamped in several different projects. Therefore, making a Left 4 Dead port to the PS3 is not a good move, because it has to be the same quality and have the same support that the other two have.
This would be difficult. Not only will valve have to build the game using the PS3's code, they also have to test it out again, which will take months to find and fix enough bugs to make it profitable. After-Release ports never really do well, I can only think of Bioshock at the moment as one that did well.
Believe me, PS3 owners, you'd be even more pissed off if Valve made a half assed port that ended up full of glitches, and never got any support. The PC version has frequent Free Play weekends, why don't you guys try it out?
*Dons hat, hops on horse, and leaves thread.*