AdMob is an ad network for web internet and mobile phone applications and they publish a metrics report every month. Until March, the #1 operating system for smartphones in the USA was the iPhone but in March 2010, Android took over, with a few percents lead.
You can find the report here and the actual graph is on page 9 of the report. The figures are based on ad requests – that is, every time someone goes to a mobile web page or app that displays an AdMob ad, AdMob adds one to the counter. AdMob is one of the biggest provider of ads for the mobile market so this gives a good snapshot of the current market.
The iPhone still has the lead on the international market but Android is slowing eating away at its share of the market.
Of course, as an Android developer, I am very pleased