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I haven't seen a difference in either traffic or Alexa rankings on our sites. Where I think, or at least hope, it makes a difference is the junk sites that have been "optimized" while in reality offering nothing of value. Don't get me wrong, intelligent SEO is a valid, even necessary thing. But those sites that just exist for no other purpose than to lure traffic without actually providing the information/product that is suggested by their google search results are a pain, and I hope Penguin further marginalizes them. Google may not give away their search engine secrets, but they give a pretty clear guide to what works. Here it is, straight from the Google horse's mouth: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=35291