Technorati does its State of the Blogosphere report each year, and there's lots of interesting detail in the 2008 edition. (I don't know when this report was issued, but I just ran across it recently.)

The link below is to the overview of the 2008 State of the Blogosphere; after that you'll find sections on who the bloggers are, the what and why of blogging, the how of blogging, blogging for profit, and how brands have entered the blogosphere.