This effect drew much attention in the webdesign community and meanwhile there are some great articles about this effect. Over at thinkvitamin is a detailed explanation of the effect. Paul Annett from clearleft gives an in-depth-view how they created the parallax effect on their silverback teaser page.
A more common explanation gives Chris Coyier over at css-tricks.com. He uses three star layers to make a cool space-effect.