Edit: Looks like this came out two days ago, sorry if this is old news.

It happened last year with L4D, and now it's happening again this year with another woefully insecure demo for public dissection. If you've played through the demo a couple of times (which would be a fine stunt considering the occasionally broken lobby system) and feel like playing as the new infected, then download this mod and gather a bunch of friends to do the things zombies do to survivors.

- First things first, once your L4D2 demo has downloaded, right click on it, go to Properties > Updates and turn off automatic updates. I guarantee you this will be patched in the days or weeks to come; you'll thank me for this later.
- Second of all, go here and download this file.
- Thirdly, unzip the file into you Left 4 Dead 2 demo 'addons' folder. That's the hard part done.
- Now, start the game.
- Go to Options > Keyboard > Allow Developer Console and change it to 'enable'.
- Press the ` or ~ key (the key next to 1 and above tab, 'tilde' on American keyboards) to open the console
- Type... plugin_allow_remote_commands 1
plugin_cvar sb_all_bot_team 1

- Start a single player game or local multiplayer lobby
- Open the console again and type "set_infected 1"

You can also enable cheats, although not in exactly the same way as before. Valve has disabled cheats directly, but you can still enable them by creating a 'listen server', whatever one of those is. Anyway, here's how you get around it:

- Open the console and type... "map e5m1..." and select the first map from the drop down menu.
- Once in game, type in "sv_cheats 1" and you're away.

You can spawn items by typing "give ..." (with a space afterwards) and selecting the appropriate item. You can spawn infected by typing "z_spawn ..." and adding the name of whichever infected you choose and there are of course all the cheats from L4D which carry over, with a whole bunch more as well. I'm going to set up a script for this to make it easier to execute and add a list of fun commands tomorrow. In the meantime, get yourself infected and start breaking bones.