This is a html/pdf guide to all the parameters of Blender 2.50's volumetric shader. It contains images illustrating what every parameter does and explains the basic of how it works.
I have been working on this guide for several days and have rendered dozens and dozens of test renders. I have just posted it on Stratus3D.com. You can read it there and download the pdf. This guide is a work in progess and I will continue to update it as I learn more.