Size of the blogosphere?

I'm preparing the class I'll give in Åbo next week and therefore I'm browsing for information on how many blogs the blogosphere actually contains these days. I see that Technorati keeps track on 54.9 million blogs and that 75.000 new blogs see the light of day every day. I suppose however loads of splogs and the like are included in these figures, I wonder where I'd get facts on the number of blogs that are maintained by individuals, groups, schools or corporations and updated on a somewhat regular basis...