Now let me preface this with saying I think women are entirely equal to men. Obviously, there are some physical differences (not that, pervert). Men are, on average, bigger and stronger than women. There are some things men can do better than women. There are, on the other hand, some things women do better than men. But, let me make my point before this tangent leads me down the wrong path...
Women CANNOT do the "what's up" head nod. At all. The nod does not work for women. It looks silly, just silly when they do the nod. Guys, and there is a difference between men and guys, but that's best left for another day, can naturally do the head nod. There is an entire language guys use. There is the "what's up" head nod. That is the one where the head goes up slightly. There is also the "good job/congratulation" head nod, which is a slight downturn of the head, no smile. There is the "Seeya later" head nod, which looks much like the "good job/congratulations" nod, only with a smile.
Guys have mastered this subtle form of communication. There was that dark era in the 80's with the point and click (think Miami Vice, or your weird uncle Stu) motion, but, after time, we have recovered from that era. Some times, the head nod is utterly appropriate because you want to greet someone, or say good bye, but are talking to somebody else. Rather than be rude to either person, you can continue your conversation and say whats up or seeya later with a slight move of the head. It's really more efficient.
Also, its 1:19 and OU sucks.