Just saw this on the official left 4 dead Blog
Get your Campaign On
June 19, 2009 - Yasser Malaika
We're pretty excited about this next Left 4 Dead update, and we hope you will be as well. Since the Left 4 Dead Authoring Tools Beta was released several weeks ago, mod authors have been cooking with gas, so to speak. The ranks of dedicated community members working hard to create new content have been swelling, and a very hungry horde awaits...
We've long believed that Left 4 Dead should fully support community content, and we've been hard at work enhancing the UI and matchmaking systems to provide full support for your creations. The latest update, which will be available via Steam next week, is focused on custom campaigns. It introduces several key features:
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.
Because Left 4 Dead is all about sharing, we've re-tooled Left 4 Dead's matchmaking system to provide extensive support for custom add-on campaigns. 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.
We're also officially releasing the Authoring tools/SDK. Thanks to all who provided feedback during the open beta, the authoring tools have been updated with a few fixes and several new additions, including:
* The tutorial_standards map has been expanded into a minimal example campaign add-on called 'Dead Line'
* A new command line utility for creating and extracting from .VPK files
* Source files for all the official campaign maps
Also, look for a new tutorial on how to package up your campaigns into add-ons on the Developer Community wiki.