One of our program managers, Richard Fricks, just had a piece posted on the Windows Vista blog talking about microphone arrays.  He describes what microphone arrays are, what they are good for, and how Windows Vista enables support for them.  If you use the built-in microphone, you should understand this feature.  It has the potential to provide a dramatic boost in the sound quality.  Expect to see more and more mic arrays appearing, especially in laptops.