Notice how in scene 12_peach, shot 21 in the BBB repository the bushes' leafs in the background appear as rectangles in the depth map. I assume this has to do with the way the bush's material is set up (probably single images of leafs arranged in various sizes and orientations).
Maybe someone more knowledgeable can shed some light on this bush material setup. And most importantly: Is there a way to get the actual leaf shapes in the depth map as well?
I downloaded the BBB production repository from https://peach.blender.org/download/ (near the end of the page). The shot in question is
scenes/12_peach/21.blend. The shot seems to link bush objects from the
envs/bushes.blend library in the BBB repository. Note that I am using Blender v2.46 for this since I could not get the shots to render properly on more recent versions. Crashes all the time. But I assume v2.46 is close to the version they were developing/using in production of the movie.