PageRank Zero Does Not Mean Google Penalty; Says Google
 

A topic most SEOs already know, but it is nice to have confirmation from Google. A Google Groups thread has Googlers responding to a Webmaster who's site dropped from a PageRank 4 to a PageRank of 0. It appears this site saw a drop in PageRank, but not a drop in rankings, because it has done Paid Review posts without a disclaimer, said Google.

http://www.seroundtable.com/archives/015714.html

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A PageRank of 0 does not always mean a charge. Sometimes, sites which joint to be punished simply absence back hyperlinks with an completely high PageRank. But if webpages of a web page which have formerly been placed well in search outcomes, instantly display the terrifying white-colored PageRank bar, and if there have not been any significant changes regarding the back hyperlinks of that web page, this implies - according to the existing viewpoint - certainly a charge by Search engines.