The first Left 4 Dead release for the Source SDK will allow the creation of custom Left 4 Dead campaigns that will be discoverable via L4D's matchmaking system. The SDK update is also due for release this spring, and is free of charge to all owners of L4D on the PC.
Read the full story. There is no mention of source code unfortunately, but then again this is only the "first" L4D release!
The 'Survival Pack' (which judging by Valve's wording could be an additional purchase even on PC will be free) will introduce two "complete campaigns for Versus", probably upgraded versions of Dead Air and Death Toll, and the titular new game mode, Survival.