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I know things are in motion for the next several days but I just got a PR6 on several sites I knew to be PR7 where their inbound links have gone up this month. Actually the inbounds that are shown but the real number of inbounds has gone up as seen on other SEs.
A rescaling is no cause for alarm but I know a lot of people on this forum will be shouting about what happened to my PR7 or PR6 site. I've personally dropped a notch but am getting loads more referrals so I'm not unhappy.
Toolbar PR is clearly counted on a log scale (I think someone guessed base 7, although I don't see how you could guess that, given the complexity of the system). Since all we have to go on is an integer, it doesn't really give you much information. For instance, for all I know, I started at low 5 and am now high 5, with 5 times as much real PR.
I think you were right on the border, and something went wrong with some of your link partners (added more links, lost PR, got banned, etc.).
Toolbar is clearly very approximate. Don't worry about it too much. How are your SERPs, that's the question!