http://www.bopnews.com/archives/000119.html#000119
Summary:
The blogsphere has a "main sequence" of blogs that get most of their traffic from outside the blogsphere, a few very large blogs that are at the center of the ecosystem, and a large and identifiable set of "bloggers blogs" that have more cachet inside the blogsphere than outside.
It's got an an easy to read graph, so it's at least as solid as a Bush budget projection...