So the Google PR update for April 2007 has started.
The PR for some websites has decreased, for some it has increased. That is the big advantage of having a large number of websites, any change does not affect you too much. You can afford to experiment. I only avoid using the Google Webmaster Tools, based on my experience it is a waste of time. You provide all data about your websites at a single place, which can conveniently accessed by anyone who wants to hack or attack your websites.
The older websites with lots of traffic have all gone to PR0, so I suppose the PR update is not complete. Others have also reported this problem.  Even otherwise I do not get much traffic from Google, so Google Search Engine Rankings do not matter.