A splitscreen mode for the PC version of Left 4 Dead seems to have been included with the release of the demo. Files with cvar's centering around the splitscreen mode were found when the dedicated server files were made available days ago. Now, a few people have figured out how to get this running on the PC.
The use of this mode on the PC does require the first player to use a Xbox 360 controller. Full details on how to get this up and running can be found at the Steampowered forums.
The use of this mode on the PC does require the first player to use a Xbox 360 controller. Full details on how to get this up and running can be found at the Steampowered forums.