Apple's iPhone and iPod Touch continue to be the platform of choice for mobile Web users, but the Google-backed Android is picking up steam, according to data from AdMob.

In its monthly "Mobile Metrics" report, AdMob analyzed what handsets and operating systems were using the most mobile data for June. While it is not a comprehensive report of the industry because it only measures Web sites and applications that use AdMob's advertising network, it does give some insight into mobile Web usage.

Apple's mobile platform remains popular, and its devices accounted for nearly 43% of all U.S. ad requests. The report also broke down requests by region, and estimated that 54% of the 45 million iPhone and iPod Touch users were in the United States. Apple continues to seek deals in new markets, and it is reportedly close to bringing the handset to the huge market of China.

While Apple's mobile devices were far and away the most popular, the Android OS appears to be gaining traction. AdMob said requests from the Linux-based OS increased 25% month over month, and its worldwide share surpassed Windows Mobile for the first time. Microsoft's mobile OS still has a far larger installed base than Google's OS, but this is an indicator that Android users are more likely to browse the Web from their handsets.

Android's quick gain in market share can be attributed to the release of more handsets, including a smartphone from Samsung. Google said it expects up to 20 Android-powered devices to be shipped by the end of the year, and handsets like the HTC Hero are already generating buzz among smartphone enthusiasts.

InformationWeek has published a 360-degree analysis of the first Android-based smartphone. Download the report here (registration required).