I have played a bit with Alien Swarm now, and after 17 hours the default Jacob's Rest campaign starts to become known. Do they plan to release other official campaigns?
As of August 2010, there are currently no other official campaigns other than Jacob's Rest. It is unknown whether or not more official ones will be released, so only speculation is possible at this point. However, the game shipped with a SDK so players could make their own maps and campaigns. If you check the game's official forums, you can find information there about custom maps and mods that players have created.
You can take a look to Swarm Armory, there are already some missions and survival maps of good quality available.