“In this tutorial I will try to summarize what I learned throught trial and error about Mental Ray shaders, and how to achieve interesting volumetric effects only through shaders. My aim is to describe how I got this image and how different parameters affect the look and behavior of the shaders. This is by no means a comprehensive guide to MR shaders but I hope it will give you some good ideas.”
http://www.cgarena.com/freestuff/tutorials/maya/shadertips/index.html