A couple of interesting hacks have appeared for the Left 4 Dead demo. People have found a way to get at the splitscreen mode, intended only for the 360 version, and more interestingly, how to play as the Infected.
It's important to note that the latter isn't a way to get Versus mode in the demo - instead it's a very choppy console-command-led method that sort of lets you at least experience what it's like to play as the Infected, albeit in a way that is nothing like the game proper.
You will be able to see the demo's levels through the eyes of a Hunter, Smoker, Boomer or Tank, and get to know their abilities and attacks, even if it's unrepresentative of how Versus really works. It works in both single and multiplayer, so you can fight against friends, but let me stress again, in a broken and unbalanced way - you can in no way get a feel for the real thing with this. Is this message clear enough? It's nothing like proper Versus.
To get Infected, follow the instructions below, which we've cribbed from Steam forumite Scuzball.
You need to make sure the Developer Console Command option is enabled in the main options, and then start a game. Pull down the console, and type in the magic words
In order to switch sides, you then need to type,
ent_fire !self setteam 3
If you're playing against AI Survivors, they'll stand still until you tell them to begin the level for themselves. That's done by telling it,
You can then choose to spawn by following the on-screen instructions, which will also tell you how to control each Infected Special. Once you've been killed, you'll have to respawn manually, using the following console commands:
So of course this means you can cheat horrifically, and for extra fun, spawn about five tanks at once and ruin the game completely. What it does is give you a feel for what's to come, but again, for goodness sakes don't try and judge Versus mode based on this abortive hack.
To get splitscreen is a bit more complicated, and requires you own a 360 controller for your PC. The instructions for that are also on the Steam forums, posted in enormous detail by user TheBusGuy.