It was just pointed out to me in this blog
Is there a Google pagerank update in November 2008?

The last Google pagerank update was on September 27, 2008. It was just a lapse of two months from July 27, 2008.

So, if we go by this trend, then, we shall see another Google pagerank on November 27, 2008.

Have they switched to a 2 month PageRank rotation/update instead of 3?

I guess we will have to wait until November 27'ish and see if another PageRank update happens or not.