http://www.l4d.com/blog/post.php?id=2599
Non-beta SDK and improved add-on support coming soon. By far the best news is:
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.
Unreal has kicked ass on the user content front for years, glad to see Valve jumping on the package train and I hope they'll expand that into future titles as well.
SDK release will also package a new bare-bones example campaign and source files for the official campaign maps. Good stuff!