Looking at the graphs below, you'll see that Android has seen significant growth over the last year and right now, the amount of traffic AdMob is seeing from Android devices is nearly level with the iPhone. Although Android has the advantage of a surge in new devices (from the likes of Motorola, Samsung and HTC), it's definitely a change from the indomitable force that was the iPhone at the same time last year.
Outside of the U.S., it would seem the iPhone is still untouchable, accounting for 50 percent of AdMob traffic. Though Android did rise from roughly three or four percent during the same period last year to past 20 percent in 2010, there's still a bit to go before it starts really threatening the iPhone.
Ars Technica repots that AdMobs uses ad requests for ads from its network as its primary metric.