For me, I believe this PR adjustment was made sometime ago and just not translated into visible Toolbar PR. Toolbar has always been a snap shot typically around 6 months old. If that is the case, then this update represents PR from that period. I noted some crazy activity over the past few months, where sites with PR0 have been ranking top 5 for very competitive terms in G. I just couldn't believe it, these newish sites getting top rankings. A thought, now that PR has dropped, a lot of people that paid for links wont want to pay anymore so they will be removed. Google then goes through see whos removed what and really sees you have been paying for the links. From this they gather the data for a major SERP update ... Just a thought.
I have read on multiple threads that PR has been obtained on blog posts dated Oct. 3, 2007 so we can assume that is the date of the snapshot? I don't think that is realistic.