Google has become more and more secretive about updates to its algorithm, despite the potentially huge effect they can have on companies’ search ranking and revenue.
Google Panda update has nothing to do with your links... but correctly mentioned that Panda targets poor, duplicate and spun content. PS: PR's have never been there for link building but getting a message across and getting noticed by journalists... Junk PR's are dead since 2011 or so... just my two cents.