Phil (over at Blogcount) has come up with a preliminary estimate of the size of the blogosphere using published counts and estimates of Blogger, LiveJournal, and DiaryLand usage. With a fudge factor, he estimates the size at between 2.4 and 2.9 million.
In this game everything is guesses and approximations, since (a) not everyone is on a centralized site, and (b) not everyone uses centralized tools like Weblogs.com. But I think the next logical step is to benchmark this number using a different technique, like the number of sites registered in Technorati, Blogdex, or even Blogshares, or a longitudinal study of Weblogs.com that looks for repeat pings and calculates a unique number of pingers.