Gottfried Hofmann shows how you can create a Cycles shader node-tree from a physical formula in an effort to create re-usable shaders from math or formulas that you may come across in textbooks or on the nerdier parts of the internet.
The new ray length node in Blender 2.64 allows to fake absorption even without a volumetric shader. This tutorial shows you how to create your own absorption shader and make it re-usable
Here, BlenderDiplom’s Gottfried Hoffmann shows how to create your very own absorption shader in Blender and make it a re-usable asset providing a background overview and showing how BLender Cycles handles light.
Gottfried also show how to translate a formula to a Blender Cycles shader, and shows some of the possibilities for creating group nodes for re-use. Check out Gottfried’s post Tutorial for Absorption in Blender Cycles here where he provides the blend-file including the finished shader.