Everyone panic! There is a Google PageRank update taking place! Okay, I am just kidding about panicing, but there is some sort of Google Toolbar PageRank update going on now.
It seems like most of the reports seem to be that their PageRank went down to a 0. So this PageRank update does somewhat seem penalty related. Others saw a smaller downgrade of their Toolbar PageRank.
It is important to note that Toolbar PageRank has no direct correlation with ranking. Google uses their own internal PageRank numbers and what they show in the Toolbar is often believed to not just be outdated but also not accurate.
I have not seen people report Toolbar PageRank increases, but I could have missed it.
The last Toolbar PageRank update was the April 2010 PageRank update.