One of the better explanations of SDN
Stumbled upon this via HighScalability:
Every time I feel like I'm "out of touch" with the hip new thing, I take a weekend to look into it. I tend to discover that the core principles are the same [...]; or you can tell they didn't learn from the previous solution and this new one misses the mark, but it'll be three years before anyone notices (because those with experience probably aren't touching it yet, and those without experience will discover the shortcomings in time.)
Yep, that explains the whole centralized control plane ruckus ;) Read also a similar musing by Ethan Banks.