I am seeing early signs and signals of a possible Google Search ranking algorithm update going on right now. It seems to have kicked off this morning and is now spiking in terms of chatter in the SEO forums and also some of the tools have picked up on it (not all are updated yet). Google has not confirmed any update, so this is one of those unconfirmed updates....
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May 13th Google Search Ranking Algorithm Update (Not Confirmed)
seroundtable.com