I just read the 27 page summary of the "Semantic Wave 2008: Industry Roadmap to Web 3.0 and Multibillion Dollar Market Opportunities" report. Here is a link to the post on this from Read/Write Web, including information about how to get the summary report:
Semantic Wave 2008 - Free Summary Report for RWW Readers
"The report defines Web 3.0 as "about representing meanings, connecting knowledge, and putting these to work in ways that make our experience of internet more relevant, useful, and enjoyable."
"The report also defines a "Web 4.0", as follows: "Web 4.0 will come later. It is about connecting intelligences in a ubiquitous Web where both people and things reason and communicate together."