Recently One Guy posted saying that he got a reply from google regarding his site loosing page rank, Dont know whether it is true or not but worth a read,

"The PageRank that is displayed in the Google Toolbar is for
entertainment purposes only. Due to repeated attempts by hackers to
access this data, Google updates the PageRank data very infrequently
because is it not secure. On average, the PR that is displayed in the
Google Toolbar is several months old. If the toolbar is showing a PR of
zero, this is because the user is visiting a new URL that hasn't been
updated in the last update. The PR that is displayed by the Google
Toolbar is not the same PR that is used to rank the webpage results so
there is no need to be concerned if your PR is displayed as zero. If a
site is showing up in the search results, it doesn't not have a real PR
of zero, the Toolbar is just out of date"

Source: forums.searchenginewatch.com/showthread.php?t=3054

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