From the official blog:
http://www.l4d.com/blog/post.php?id=2390

This is part of the L4D SDK rollout, which is currently in a closed beta and will be released shortly after the launch of the L4D Survival Pack.

To support the SDK, we are extending the matchmaking system to allow for third party campaigns. We will be adding extra features so players will be able to find dedicated servers running the campaigns they have installed. They will also be able to see friends playing third party campaigns, and be directed to download them as necessary. Another major feature we are planning to add to the matchmaking system at a future date is team-based Versus matching. This will allow two teams of four to locate each other and compete in a Versus match.

We are also revamping our player matching based on skill level. Currently the system tries to match people with the same skill level and best pings together. We will be revisiting this system to tweak it based on data we have collected.
This should answer many questions regarding just how they were going to work out the lobby system with new campaigns. There is also an interesting titbit of information which mentions "team versus matchmaking". A brilliant addition, considering the amount of time I spent trying to play a game of VS with a friend.