Yahn Bernier, a Valve employee, on the SteamPowered forums writes,
"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."
Split-screen is coming to the PC version of Left 4 Dead, but as an "unsupported bonus feature." The optimal configuration will be two joysticks, but one keyboard and mouse and one joystick will be possible; two keyboard and mouses will likely not be supported.