[Charts via Ars Technica ]
AdMob (which we'd be remiss to forget, is owned  by Google ) is the biggest mobile ad platform on the planet, so its data measures a very specific thing. Not total number of handsets, or most money spent, or biggest app store, but pure mobile Web use. And in that sense, there are really only two competitors in the U.S., and one is about to overtake the other.
Back in October 2008, the iPhone, BlackBerry, and Microsoft  Windows Mobile platforms were all competitive in mobile Web use. But consumers realized quickly enough that iPhone was the only real consumer platform--BlackBerry and Windows Mobile were geared toward enterprise use, which doesn't mean a whole lot of casual web browsing. The consumers who wanted more entertainment and browsing switched to iPhone, and the iPhone's mobile Web use shot up, while BlackBerry and WinMo sunk.
Android had been steadily rising since 2008, but it was the release of the Droid (and its massive, iPhone-beating sales) that really took it past the business smartphones and up into the top two--and now, given Android's uptick and iPhone's downtick, it looks like within the next month or two, Android will pass iPhone in mobile Web use. And with all the buzz about new Android phones like the groundbreaking HTC Evo 4G , Apple  is going to have to bust out some seriously impressive stuff with this year's iPhone 4.
Of note is the worldwide standings, pictured below, which tell a different story. Instead of being a challenger, Android is still the underdog worldwide, though rising fast, and the iPhone controls so much of the market that it's not really in danger. Symbian also makes an appearance; though it's pretty much a non-starter in the US, Symbian has the largest userbase in the world, though it's clear that in mobile web browsing, the Nokia OS is fading fast.