Alan, was wondering the same thing
so did a little testing with it, it seems that the only mic that works on my 4S is the lower one in waze mode.
Putting your finger over the top one and activating the voice commands, it hears everything I say, but when holding fingers over the lower one it keeps asking me to repeat myself.
Area Manager: US - SW Missouri - Ash Grove, Willard, Republic, Springfiled and surrounding area.
iPhone 4S - AT&T - iOS 5.1 (9B179)