I dont recall when, but at some point when I was working with my mountain scene I managed to break the renderer node. It renders these dark areas everywhere and I dont find anything wrong with my settings. First thought I had, was that there is something wrong with my terrain but eventually concluded that the render node has somehow become corrupted. Everything renders fine when I created a new one....
On the Advanced tab, ray detail region was set to "no detail". When this is set back to "detail in crop region" or "detail in camera" it renders as normal. This setting affects the detail of the polygons that cast shadows, ray traced reflections and global illumination.
Matt
Oh ****. I dont even remember touching that, silly me. I probably tried every setting but but that :-[