No, the SDK has no been released yet as most of us know, but the documents/tutorials made by VALVe have been released. They can be found here: http://www.l4dmods.com/index.php?option=com_wrapper&view=wrapper&Itemid=71 and http://developer.valvesoftware.com/wiki/Left_4_Dead_Level_Creation Both links are basically the same, it's just a matter of which one you're more comfortable looking at But looking through them, they seem quite helpful to people like me who have never mapped for the source engine.
Cool, can't wait for people to start making L4D maps with this they can learn how to properly make it.