Speaking on the Steam forums, Valve designer Yahn Bernier has confirmed that split-screen play is planned to be included in the final version of Left 4 Dead, despite the fact the latest demo patch has removed the feature for PC players.
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 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.