i do believe, there was a PR update at the end of August! The reasons for this believe is simple:
A domain, I have published in early June moved from grey to white. Any other values (of existing PR) did not change in that period, which brings me to the following thoughts of this "update":
It looks like a 2-step update to incorporate the problem (solution) of bought links from Google.
First they say: "lets clean up PR values based on the feedback we got and the automated filters we have implemented". Remember, that Matt requested to report bought links. I do not believe, that they take these reports into filters blindly, rather match those with the automated results they calculated to find "highly likely" matches and cross reference those with other calculations...
For that reason, this "update" was the clean-up of internal PageRank calculations to flag "link sellers" and "link buyers" alike and introduce some new damping factors for the coming update (which would happen after Christmas in that theory).
Internally they are calculating with the new factors and the SERPS will IMHO incorporate these changes now slowly within the next weeks.
Also I see a strong change in the freshbot calculations. If you get a PR8 link today, you are listed extremely quickly for competitive terms, but your star falls also extremely quickly in the days after. That leads me to the thought, that the (internal) PR value is now stronger dependent on the time span you have that link.
However, I believe there was a clean-up update, otherwise the status would not have changed from grey to white on domains 4 weeks ago!