I hope that this is not a mistake. Still, chances are it is. We know that Valve has gone on the record as saying that they will not develop for the PS3, but anything is possible.
As much as I would like this to be real, my guess is that EA just messed up. Still, one can dream:
Found it on NeoGAF.
Update: Looks like my dreaming wasn’t enough to bring L4D2 to the PS3. EA has since removed any mention of a Left 4 Dead 2 for the PlayStation 3.