There is discussion at a DigitalPoint Forums thread that Google is updating the visible toolbar PageRank scores.
I personally did not look deep into this. Because (1) Toolbar PageRank is not all that significant and (2) I am very under the weather.
Forum discussion at DigitalPoint Forums.

Comments:
gabs
12/17/2007 02:35 pm
Yep.. round 2 ... Another pr7 to a pr4 WOW....
Andy Beard
12/17/2007 03:09 pm
Can't really call it round 2 if there have been at least 5 previous rounds since the beginning of October.
gabs
12/17/2007 03:57 pm
Sorry Just my dumb expression.. My intention wasn't ment to misslead..
Matt Cutts
12/17/2007 07:06 pm
Looking over that thread, it looks more like one person said that their PR4 went to PR0? The first sentence I saw on the PR0 blog was "Looking for notebook batteries, cell phone batteries, and external batteries for all your consumer electronics." Other posts are marked as sold yet the links flow PageRank. So it's just this blog isn't trusted as much in Google; it's not an actual PageRank update.
gabs
12/18/2007 07:21 am
Matt have you got a name for this then? I know other sites that dropped over the w/e.. So if its not a "update" is it just a "penalty" ?
Steven Helms
12/29/2007 08:20 pm
My PR4 just went to PR0. Is this another version of the "Google Dance"?