I just conducted a tiny experiment with the original headset that comes with Nexus One: I plugged it into the phone, expecting the phone built-in microphones to be switched off, replaced by the microphone on the headset.
I was disappointed to see that the voice input behavior of the system didn't change: The phone seems to continue picking up sound from the built-in microphone(s).
So, I thoroughly covered the headset's microphone with tape and tried again...
Still, no change. The Nexus One's built-in microphone are fully enabled.
This is disappointing because I really need to disable the Nexus One's built-in mics, so that I can use a different microphone instead.
Is there a way to accomplish that?