The Left 4 Dead blog
has some news about the upcoming patch for next week. With the authoring tools coming out of beta next week as well, we'll have the ability to join custom campaigns right from the menu system. It will automatically download the needed files when you try to join the custom campaign. If you have some custom campaigns installed on your computer you can actually choose to play that one and find servers that are hosting it at that time. I wish there was a way that you could rate the campaigns though so you can weed out the good and bad ones to join.