This Blender 2.67 Cycles Tutorial explains the fac value of mix shader node of cycles it's usage in context of Lightpath Node. The fac value takes a decimal number which can range from 0 to 1, but in case of LightPath info node, the value becomes boolean which tends to confuse a lot of people.
This Blender 2.65 Cycles Tutorial illustrates How to create invisible Mesh Lights using Light Path Node, 'Is Camera Ray' as well the Ray visibility options in Object Properties. It also explains the pros and cons of each method
This Blender 2.65 Cycles Tutorial illustrates How to isolate objects on shadeless white background using cycles Material node setup for World. It also describes a node setup to cast reflection on objects while world appears white to the camera using Light Path Node.
This Blender 2.65 Tutorial will help in Understanding the concept of holdout shader. Holdout material, as the name suggests, holds out the surface from being exposed to the camera - and will dig in deep through other surfaces too - thereby creating holes in the image.