This is the nature of Z depth files, they cannot be antialiased. All programs have this same problem. That is why you will never see 3d motion blurred depth map shadows because they cant be anti aliased. This is why anything that is a post effect like fur, paintfx and optical fx will have slight aliased edges where they go behind something.
Compositing apps like shake have an option to blur the zdepth file so you can cleanup putting one object in front of another with zdepth compositing. I usually try to stay away from zdepth compositing anything that overlap other objects especially with film since a small aliased edge will turn into a a 6 inch ugly edge on a 60ft screen. But for television blurring the z will usually work fine. Otherwise just stick with rendering more layers instead of using zdepth compositing or using a zdepth shader.
Deke
