Matt Cutts says w3c validation is not a factor for ranking in Google, he has also mentioned earlier that w3c validation isn't in the algorithm. I hope this end such argument that often being discussed in different forums.
w3c validation was indeed considered as a search engine ranking factor before Matt Cutts cleared this doubt. I think this should clear all the arguments search engines expert had regarding it's importance for search engine rankings.
w3c validation and ranking factor
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