AdMob's March Metrics release compares Android and the iPhone across a variety of measures:

There is no doubt that Android is growing fast and has made an impact since its launch.  The HTC Dream (G1) generates 2% of our US requests and is now the #4 smartphone behind the iPhone, Blackberry Curve, and Blackberry Pearl.   Android growth accelerated after our launch of ad units in apps at the end of January, which likely reflects strong consumer usage of applications on the Android platform.

However the question we get most often is – how does Android’s growth compare to iPhone’s growth at launch?  Our best answer: both grew more quickly than the market, but iPhone growth was much faster than Android.  To put some numbers to this, we found that Android traffic in the US grew an average of 47% per month since it launched five months ago while iPhone traffic in the US grew an average of 88% per month in the five months following the launch of their App Store. See the report for more details on the comparison.

Android facts from AdMob:

• The G1 is now the number 4 smartphone in the US trailing the iPhone, Blackberry Curve, and Blackberry Pearl.
• The HTC Dream (G1) was the number 10 device in the US with 2.0% overall share of requests in March. 
• Android has captured 6% share of the smartphone Operating System (OS) market in the US and is now tied with Palm as the fourth largest smartphone OS behind iPhone, RIM, and Windows Mobile.
• Requests from the G1 increased 2.6 times from January to March following AdMob's launch of ads in applications.

Remember that these data reflect traffic and activity on AdMob's network and are not necessarily exactly representative of the larger mobile Internet as a whole (although directionally they are):

 Global handset data (share of requests on AdMob network):

US handset data

UK handset data

___

Related: T-Mobile has sold 1 million G1 phones since October. The G2/Magic is coming to Vodafone in May in the UK and probably later this year to T-Mobile in the US.