The SDK is currently in closed beta and will be released after the fairly imminent Survival Pack. Along with the SDK the matchmaking process is being extended to:
* allow for 3rd party campaigns
Valve then plans to add extra features such as:
* find servers running the campaigns that you have installed
* see friends playing on 3rd party campaigns and download those campaigns
An interesting planned feature is to enable team based matchmaking:
* two teams of four players join up to compete in a Versus game.
Skill based matchmaking is also being refined to better try and match players of similar skill who have good pings together.
The blog post detailing these changes is here: http://www.l4d.com/blog/post.php?id=2390