The Technorati Weblog has an interesting article On Blogosphere Growth. It looks like Technorati's index is doubling in size every 5.5 months! An other interesting mesure is the number of posts: ... about 50.000 per hour.
An other interesting point is that tagging is used more and more. If you ask me, we have to thank the new tools that make tagging so much easier, like Wordpress using "categories" as tags in feeds. (And I guess social bookmarking services like del.icio.us make Technorati's life much easier).