Hai Zhuge has an article on Discovery of Knowledge Flow in Science (pdf) in the May 2006 issue of Communications of the ACM. Zhuge focuses on the scientific citation network that is a familiar topic in academic circles. I found it interesting for the discussion of knowledge flow and knowledge spirals, as this applies to just about anywhere you can identify knowledge flows, like in blogs.
The basic idea is that, people learn from and build upon the ideas presented by others. When these people then publish and reference their sources-of-inspiration, one can visualize the flow of knowledge (or ideas) over time. Once can also get a sense of who are the big thinkers and a sense of community of like-thinkers. I like Zhuge's analogy of "knowledge energy" that suggests how some research centers appear to create and pass along more knowledge than others.
There are obviously links to the social network analysis community, and I am fairly sure there has been SNA work that looks at citation networks. Zhuge touches on the idea of roles within the knowledge flows, with the focus being on how those roles relate to the citation network: source (frequently cited), authority (many citations, in and out), bee (diverse citations), hub (many outward citations), and novice.