I didn't know any kind of nod existed until I moved to NC from MI. I asked my parents if they knew all of these random strangers who were nodding/smiling/speaking and they explained it as a southern thing.
Since then I've experienced the Black Nod, the Natural/Curly Nod, the Professional Nod, the Biker Wave, etc. Folks greet each other when they see someone they identify with.
Blame it on the cell phone...