From my experience, the integrated microphone is a little on the quiet side. Right now I have a 11" MacBook Air 2011, and I've had several FaceTime, Skype, and Facebook video chats with friends, and I noticed that they asked me to speak a little louder several times because I didn't sound loud enough.
Before this computer, I had a 13" MacBook Pro 2009, and I was never asked to speak a little louder, but I'm sure the microphone placement has a lot to do with it as well. On the MacBook Pro, the mic was placed above the keyboard around the escape key area, but on this MacBook Air, the microphone is placed on the left-hand side of the machine right beside the USB port and headphone jack input.