Matt Cutts says in a comment over on a pretty interesting post at SEOmoz – we did do a full PageRank update several weeks ago – there's less PageRank flowing around in some areas (e.g. search and SEO). Vanessa Fox's site dropped by one as well, and for her as well, it's just a case where less PageRank is flowing in some niches of the net. PageRank doesn't always monotonically increase.
There's lots of other good info from Matt over there too, but that one bit about there being less PR flowing through the SEO niche got me wondering…
Some people have been saying that the Toolbar PR penalties we recently saw were not "real". In other words, they were for show only, but didn't reflect real behind-the-scenes PR. I think those people might be wrong. If the PR losses were real for all the SEO folks who had link-selling PR penalties applied, then those losses would likely affect the flow of PR across the SEO niche. And that would also explain why the SEO niche in particular would have less PR flowing through it, as opposed to every other niche out there.
I certainly concede that I could be wrong, but it seemed like something that should be thrown out there as a thought for an early Sunday morning.