Mac OS X Hints reports on “a new and seemingly little-mentioned feature” in Mac OS X for Intel Macs:
It appears that the ambient noise reduction feature is designed to remove the background noise that the microphone would otherwise pick up. It should make a difference in any app in which you use the microphone, making your voice clearer while diminishing unwanted noises.
Just go to System Preferences–>Sound to enable this setting. I still need to fiddle around with the input volume to get the most out of the feature, but the effects are already pretty noticeable using the built-in microphone on my MacBook Pro: