I haven't really heard a peep about it yet either, but based on their developmental history surrounding L4D1 and their lack of DLC content to expand the game before jumping strait into v1.5 (L4D2)... I'm guessing L4D3 will be here sooner rather than later.
Now that Portal 2 has been released, they should be rushing it. I'm surprised nothing was announced in E3, frankly. But TGS is right around the corner. And after their last announcement for their recent L4D game, they will probably keep it somewhat under wraps
until the last possible moment to minimize petitions and hate mail.