After the latest update, the splitscreen mode was removed from the demo in the PC version of Left 4 Dead. A few of the more vocal members of the Left 4 Dead community took to the Steampowered forums to voice their "concern" over the matter. Fortunately, Yahn Bernier, a staff member at Valve stepped in to let everybody know that the splitscreen will return eventually.
So splitscreen was always something we only wanted to officially support on the x360 due to the controller issues. However, we are planning on making some of the functionality work on the PC as an "unsupported" bonus feature. We turned it off for the demo because we didn't want to have to deal with any additional bug reports etc. about it interfering with the demo experience, but the current plan is to re-enable it after we launch the full product.There you have it ladies and gentlemen, a confirmation that the splitscreen mode will return at some point for the PC version of Left 4 Dead. That's certainly not too bad for a feature that was never announced as coming to the PC version to begin with.
There are a number of tuning variables, the but tricky thing will be the controller. When testing, the best setup on the PC is to have two joystick/x360 style USB controllers. I think it will also be possible to have one keyboard/mouse and one joystick player.
I haven't looked into supporting two keyboards/mice, as I'm not even sure PC's can deal with that and I doubt we're talking about lots and lots of possible users for such a config.