GREAT tip from Tidbits that points out something I (and I bet many of us) didn’t know:

Facetime uses the microphone at the top of the iPhone that is normally dedicated to noise-cancellation.

You probably know that there’s a speaker and a microphone at the bottom of the iPhone used for most calls, but when you put the iPhone into speakerphone or Facetime mode, the device uses its proximity sensor to tell you’re not holding the phone next to your face and it switches which microphone it uses to avoid feedback.

Genius!