It's always difficult to accurate data on a timely basis without being a subscribing industry insider.
So - here's a snapshot of almost a year ago, follow the slideshow - AdMob Metrics
However - I think it's always interesting to see the reported browser types - here's who is dominating what in what parts of the world - http://1.bp.blogspot.com/-57b953A7YH8/TVmUYaohPBI/AAAAAAAAAWE/EcKeM2cVgfg/s1600/mobilebrowser.png
So - here's a snapshot of almost a year ago, follow the slideshow - AdMob Metrics
However - I think it's always interesting to see the reported browser types - here's who is dominating what in what parts of the world - http://1.bp.blogspot.com/-57b953A7YH8/TVmUYaohPBI/AAAAAAAAAWE/EcKeM2cVgfg/s1600/mobilebrowser.png