Some more news has been released via the Left 4 Dead blog about how exactly the new user created custom content is going to be implemented into the game. This will all be done by using an add-on platform and the new VPK file system.
Valve wrote: Third party campaigns are designed to be installed using a new Add-On framework. Content authors will be able to package up their new maps, along with new posters, models, and textures, into a single .VPK file. To install and activate this content in-game, players need simply download the .VPK and double click on it. Alternatively, they will be able to drop it into a new "addons" folder. The list of installed add-ons will be viewable from a new item on the 'Extras' main menu flyout.
It's nice to see that Valve has decided to make installing and playing custom content as easy as possible, and with the update to the matchmaking system if players do not own the VPK files needed for the custom campaign then it will be offered to them to download.
Valve wrote: When creating or searching for a lobby, a new option named "Add-on campaign" will let you select from among the add-on campaigns that you have installed. You can then find games or lobbies as well as create a lobby for that campaign. You can invite your friends, too. If they do not have the campaign installed, they will automatically be offered and an option to download it. We've also made it easier to create lobbies by adding a 'Create Lobby' option directly from the 'Play Online' menu. Once you're ready to go, the new matchmaking system will automatically try to find any dedicated servers nearby that have the custom add-on campaign installed, or you can make your own local server.
On my quest for answers on what exactly the VPK file is I managed to find that everyone's favourite Source SDK development program GCFScape now supports VPK files too. It turns out VPK is just a another variation on the GCF file that populates most Valve games, so being able to use others work looks like its possible, not sure what the copyright would be on that either. The last piece of news from this update is that as of this patch going live the L4D Authoring tools/SDK will go live as well. I would still like Valve to give modders access to a full SDK for L4D as the director in modding hands could see some great things be developed, but as of right now it does not look like it's going to happen. A shame really.