So far, this is what I've been able to figure out using Google:

* Team Blue = Knowing that you're having a boy
* Team Pink = Knowing that you're having a girl
* Team Green = Not knowing if you're having a boy or girl

(Apparently there's even a Team Purple, where you know you're having fraternal boy/girls!)

But anyway, back to my question. Where did the phrase "Team Green" come from? And why Green?