A DigitalPoint Forums thread is reporting yet another large PageRank update taking place in the Google Toolbar.
So you know, there already was a PageRank update this month and it is unusual for there to be large PageRank updates twice in a month.
Some suspect that this update is again attacking paid links.
Forum discussion at DigitalPoint Forums.
Update: I posted a longer piece on this at Search Engine Land named Google's PageRank Update Goes After Paid Links?