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Well what if I show Page A to IE users, Page B to Firefox users and Page C to Opera users. Since IE retains the most market share, should Google orient their rankings to IE? How should they present their user agent to the webmaster?
IMO, in an ideal world, they would spider a page separately with the top 3 browser's UAs and provide different rankings depending on the UA. If you google a site with IE, you get the IE rankings. Google a site with FF, you get the FF rankings.
As an aside, one of the things that makes a SE's life more difficult is having to visit robots.txt before visiting a site. They have to announce who they are when visiting a site and obey the robots.txt. If they spider a site without visiting robots.txt first, they are badly behaved. But following the protocol enables the webmaster to more effectively show the SE different content than the browsers.