Marc Andreessen has a good post explaining OpenSocial from a tech angle http://blog.pmarca.com/2007/10/open-social-a-n.html
Marketing angle: http://www.christopherspenn.com/2007/10/31/google-opensocial-and-marketing/
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Disclaimer: This blog reflects my personal views only. Content does not represent the views of my employer, Info-Tech Research Group.