According to TechCrunch:

“When it comes to the mobile Web, increasingly there are only two mobile platforms which matter: Apple and Android. According to AdMob’s October, 2009 mobile metrics report, the iPhone/iPod Touch and Android phones accounted for 75 percent of mobile Web traffic in the U.S., as measured by all the mobile ad requests it tracks. That number is up from a combined 65 percent in September, 2009.”