Swinging this conversation swiftly back onto topic.

The z-depth channel can be used only in iff files (mayas own format). As a test turn it on in the globals and then render as an iff. Open the resulting file in fcheck and then push z you should see the depth info.

As for using it in aftereffects I can't tell you as I dont use it. However there is an adobe plugin that allows you to view iff's in photoshop and seeing as photoshop is so closely linked to aftereffects it should probably work with that too. You could then pull that channel out and use it as required.

Alan