I have used mist pass to achieve a psuedo Z depth value for rendering a Depth Of Field effect across transparent objects or planes. As the regular Z depth value is mesh based it only delivered object shapes for blurring. But using the mist render pass as the Z value means you can see through a mesh.
http://blenderartists.org/forum/showthread.php?t=177004
Sorry for link but .blends are uploaded there already.