Although Maya 2009 has a new render pass system for MR but many people are having problem with it. If you are one of them or you're using Maya 2008, there's a good news for you. Luma Pictures released their in-house render pass system, Nexus, for public use. It uses MR's frame buffers to render out the passes which means you can render out all passes with one render. With this shader, now you don't to fuse with frame buffers on your own (which is a painful job).
Check it out here:
http://luma-pictures.com/nexus/