The new MacBook Air (Jun 2013) comes with dual microphones. What's the advantage over single microphone? Anything to do with background noise cancellation?
You are correct. The signals from the two microphones are used, in conjunction with the Mac's built-in digital signal processing software, to cancel background noise for a person speaking into the microphones, for use in webcame programs like Messages and Skype. The signal thus generated is monaural; the two microphones cannot be used for stereo recording.