It highly possibility that google update will happen in this week based last three month update: 5th February 2013 7th November 2012 2nd August 2012 I think do not purchase any domain within this week until google update done.
2 factors have already been identified http://www.panfecta.com/google-update-may-2013 and more to come
I think by now just about everyone is waiting for the next update. I agree that according to the timeframe, the next PR update should be occurring any day now.