There are currently 27 million iPhones and 12 million Android phones in use around the world

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Source: AdMob April 2010 Mobile Metrics Report, 26th May 2010
Notes:
1 - "In the April Mobile Metrics report we take a look at unique Android and iPhone devices in our network. The numbers represent the unique devices that requested at least one ad from the AdMob network in April 2010. Please note these are not market estimates, rather data from our network that could be used to inform relative comparisons between the platforms."
2 - "Keep in mind that these unique device numbers are from the AdMob network only and reflect the adoption of our products and business operations. We don’t know what percentage of the total universe of iPhone and Android devices AdMob reaches. However we believe the data is useful on a relative basis given the large sample size of devices in our network."