A very interesting question indeed. This is the subject of this 10 pages paper : How can we measure the influence of the blogosphere?

Another paper to read : Implicit Structure and the Dynamics of Blogspace
"Weblogs link together in a complex structure through which new ideas and discourse can flow. Such a structure is ideal for the study of the propagation of information. In this paper we describe general categories of information epidemics and create a tool to infer and visualize the paths specific infections take through the network."

Want another one? How about BlogPulse: Automated Trend Discovery for Weblogs

All these links are from Workshop on the Weblogging Ecosystem: Aggregation, Analysis and Dynamics.