Bob Wyman is reporting that PubSub is watching 8 million blogs. About 4 million are active, according to him, although I couldn't quickly find a definition of active. Even more interesting is that the gap between number of blogs and overall output is increasing, according to this chart, and growth in output is relatively slow.
PubSub is a tool worthy of much more utilization and promotion in the blogosphere. Since it doesn't tie in neatly to publishing tools, the service, AFAIK, doesn't get nearly the press it deserves. But let's say you wanted to watch the entire blogosphere for links into BoingBoing, or heck, even your own blog, in real time. Where would you turn?
You might say Technorati, but they're only advertising 6 Mil blogs watched and they're trying to be a search engine on top of a watch engine. Plus polling RSS just isn't the right model for watching, even though PubSub does support such an interface.
PubSub might be the best kept secret for keeping an eye on the blogosphere.