I've noticed this year that all the QBs in the NFL all seem to have a round green dot on the rear of their helmets - what is this for? Is it new? Does it have something to do with the radio receiver in their helmets? No other position seems to have this green dot.

Yes, the dot siginifies a helmet with a raido receiver in it. There can only be one green dot on the field at a time.

Cool! I learned something new today! Thanks.

Here, for more info: Green Dots on NFL Quarterbacks' Helmets