Android's mobile web traffic overtook Apple's mobile web traffic in the US in March 2010

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"According to the report, Android traffic in the U.S. increased to 46 percent of operating system share compared to 39 percent for the iPhone operating system. Worldwide, the iPhone OS still took the top spot, taking 46 percent of the OS share, with Android phones seeing 25 percent of impressions. AdMob measures mobile ad impressions, which is a proxy for overall traffic."
Source: AdMob's March 2010 Mobile Metrics Report, reported by Techcrunch, 27th April 2010