As promised, here is the .blend and the slides from the Cycles Light Path Magic talk at the Blender Conference 2015 in Amsterdam. Which was about the light path node in Cycles.
Thank you all for showing up and being such a great audience. It was a lot of fun to present this topic to you, I hope whatever topic I choose gets accepted next time as well. Also my thanks to everybody involved in the planning and realization of the event, you are all contributing to make the Blender Conference the unforgettable event it is. See you all next year (the latest).
The Light Path Node is also explained in depth in our book, The Cycles Encyclopedia.