Total mobile advertising on Android phones in the US rose by 690% between January and June 2010

"Android ad requests grew 47% month-over-month. Since January, Android has grown 690%.
iOS ad requests increased 24% month-over-month. Since January, iOS has increased 15%.
RIM ad requests increased 18% month-over-month. Since January, RIM has increased 66%.
iPad ad requests grew 327% month-over-month."
Source:  Data from the mobile ad network Millennial Media, reported by Fortune, 18th August 2010
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Note - Millennial Media is only one network, but it's figures are likely to be indicative of the overall picture.