Well, I got my Atrix, and I've made several calls with Tango with no problems.
I did look at the front of the phone and the mic is right on the front edge, at the very bottom of the screen, just at the lip of the frame.
It's a tiny hole. Someone would have to have a good chunk of their palm on the bottom front of the phone. Holding it upright, out in front of you, with the base resting in the palm for a video call, I can see it being possible. But, you'd have to have huge, ape hands (Like MINE!).