I dont think this is in Google way of thinking as they always seem to take the view that a website should optimised for the long term and not specifically for the ranking updates.
Our online backup site (http://www.securidata.co.uk) was showing PR5 for the first time a couple of days ago ( lated for a day) but has since dropped back to 4.
I wonder if this is a pre-indicator of the next page rank is shortly to arrive. It would sort of fit with the 3 month time frame of recent updates.