IMHO, the Cluetrain Manifesto is still the most visionary book, indeed a feast of recognition, of how the Internet is accelerating the shift from broadcast to search, from push to pull, from controlled messaging to open conversation.
From where I stand, the three most relevant themes in the Cluetrain are:

1. The significance of conversations, and how the Internet is bringing them back.
2. Why and how businesses need to change as a result.
3. The power of storytelling.

Below is my collection of references from the book, with an emphasis on the first theme, the rebirth of conversations.
[UPDATE, July 29, 2007: Doc Searls refers to my excerpts from the cluetrain on The Doc Searls Weblog: Saturday, July 28, 2007. In response, I submit that we should combine the teachings of the Cluetrain and the Long Tail theory to be able to engage in the conversations that matter most to us.]

Jim says: The exceprts from the Cluetrain Manifesto are key to understanding the power of Virtual Cafe approach to conversation. One of the key plays in this field is Doc Searls, a legendary thinker and doer. See: www.doc.searls.com

I paraphrase one of Doc Searl's gems: "There was the high level executive in a major corporation. The Cluetrain stopped at his door four times a day, but he never got onboard."

Please read some of the excepts; you will be impressed: http://www.josschuurmans.com/2007/07/the-cluetrain-m.html