According to AdMob’s March 2010 Mobile Metrics Report (PDF Link), the iPhone 3GS and second-generation iPod touch combine to generate more than 60% of worldwide iPhone OS traffic on the AdMob network. The iPhone 3GS is the most popular iPhone OS device worldwide, representing 39% of iPhone OS traffic, followed by the second-generation iPod touch with 25%, the iPhone 3G with 20%, and the third-generation iPod touch with 12%. Notably, the first-generation iPhone and iPod touch represent only 4% of worldwide iPhone OS traffic share combined, or 2% each. In terms of iPhone OS version distribution, the two most recent versions of the software dominate, with iPhone OS 3.1.3 accounting for 44% of worldwide iPhone traffic, and iPhone OS 3.1.2 following closely behind with 42%. AdMob’s metrics are based on its advertising network of more than 18,000 mobile websites and applications around the world; in March 2010, AdMob received 6.1 billion requests from iPhone OS devices.