Google’s Android smartphone OS has been growing at a rather impressive rate in recent months. According to an AdMob report that examined mobile metrics from March 2009, Android traffic in the U.S. grew at an average rate of 47 percent per month. Still, despite the solid growth, Android substantially trails the iPhone, which has grown at a staggering rate of 88 percent per month in the five months following the launch of the App Store.

The report reveals the disparity in traffic (and growth) between the two phones: the iPhone received eight times more traffic than Android, with 607 million iPhone requests to Android’s 72 million. 13.3 percent of handset requests come from the iPhone and 8.8 percent from the iPod Touch. Together, the iPhone and iPod Touch totaled 1,657 million requests worldwide. While Android isn’t out of the running, it has a lot of ground to make up in the mobile market. Thus far, the HTC Dream (G1) generates only 2 percent of U.S. requests.

In terms of overall requests, Android is the fourth most popular smartphone, behind the iPhone, Blackberry Curve, and Blackberry Pearl. The Android OS comprises 6 percent of smartphone operating systems in the United States. For mobile operating systems, it ranks fourth (along with Palm), trailing the iPhone, RIM, and Windows Mobile.

The full report can be found here.