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Rankings are a lot less stable than they used to be, so I think those days of cleanly seeing differences across data centers are pretty much gone. This is something I deal with a lot in the MozCast project, and today's ranking, even across short time periods on a single data center are pretty volatile, especially for keywords with a news/QDF component. I wrote a bit about it a while back: http://moz.com/blog/a-week-in-the-life-of-3-keywords Sometimes, different data centers will show things like features that are in testing and they may have regional differences for locally sensitive queries, but the index itself seems to propagate pretty quickly these days.