There are a number of YouTube videos that I have found to be very helpful in understanding not only the core ideas behind SDN but also it's history. In particular the following videos featuring Proj. Scott Shenker and Martin Casado, two of the pioneers of SDN:
Stanford Seminar - Software-Defined Networking at the Crossroads:
http://www.youtube.com/watch?v=WabdXYzCAOU
An attempt to motivate and clarify Software-Defined Networking (SDN)
http://www.youtube.com/watch?v=WVs7Pc99S7w
Origins and Evolution of OpenFlow/SDN - Martin Casado
http://www.youtube.com/watch?v=4Cb91JT-Xb4