Earlier quoted context omitted.
From panels/ISPs, -OR- You insert some js in your pages, and it's "quantified". They actually have some pretty cool stats, and exact numbers unlike alexa.
problem 1: phones don't do js problem 2: phones don't have panels problem 3: phones don't connect to isp's they connect to carrier wap gateways. One attempt to track mobile is from Opera but it's highly skewed to the type of people that have Opera on their phones and the activity that's significant for them: http://www.opera.com/mobile_report/
The iPhone is already the best selling phone in the US.