Speaking of L4D...

The official blog has some useful information in the latest post:

Add-Ons
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.

Matchmaking
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 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.

There's also some info on the release build of the SDK.