Green screen video on top of web site.. how?

How are they accomplishing this small video?

http://www.websitetalkingheads.com/index.php

I know they are green screening a video, but what format are they using in flash?

It's an .flv using the On2 VP6 codec. It's all built into Adobe's Media Encoder that comes with Flash, you just check the "encode alpha channel" box.