Group Blogs, Blog Communities
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I think he's onto something, but I think the terminology is wrong (Dave's comment about using RSS to dynamically assemble group blogs by category is also interesting).
There's a difference between a blog aggregation, which I would call a "blog community", such as JavaBlogs or the Daily Kos diaries and a "group blog". A blog community brings together content from people with related interests. They tend to be insular and mostly read by the other members of the blog community. It a slightly more distributed discussion forum. A group blog is when a couple of people come together on one blog to write about the same topic, for example En Banc (legal), or Not Geniuses (politics). The distinction may seem pedantic, but I think it is important. A group blog is not insular; it is a part of the larger blogosphere. Group bloggers are often experts or well-reknowned in their own right before the join together. A group blog allows people who might not have enough time to run their own site a chance to create something that's bigger than the sum of its parts.
(P.S.: I am writing for a group blog which I started. Currently, there is one other author, but as the traffic to the site heats up and the 2004 campaign gets more underway, I am going to need more help, because I won't be able to keep up with the time demands of indexing every campaign ad.)