PR update doesn't make any changes in SERPs. If you saw changes in PR change, than PR is rolling out right now. If you saw changes in SERP, that doesn't have to do ANYTHING with Google PR.
It seems that another Google Panda algorithm update was rolled out last week, and it marks the final manual Panda update from Google. A Google algorithm update is a change in the algorithm that is used to determine the page rank of sites on Google search – in other words it is an added filter that tries to prevent poor quality sites from showing up in search results.
Running after Pr update to gain help in Serp's is not going to help. Instead build authority of your site and move forward with solid intent. Page rank is for sellers who scams newbie's by selling links and putting their sites into hard times. Also there is a Sticky thread anything related to page rank update is rolling down should be posted there.