provides the best overview of the Content Pipeline made available so far. He also added links to a demo of the pipeline that he gave at Gamefest. Thanks Michael!
Shawn Hargreaves also put out a collection of more informal musings on the Content Pipeline. If the density of Michael's post gives you the shivers, walk through Shawn's posts and then give it another go.
However, as Shawn notes, the Content Pipeline won't be released as part of the initial beta. This information is being made available to give us all a better feeling for the motivations of the XNA team and to get a sense of where they're going.