Surveys are used to track the market share of different operating systems in a country. According to the latest survey by ComScore, Google's Android OS has 52.2% market penetration in the US, while the iOS sits on the second spot with 33.4% of the market and the other 15% is dominated by Feature phones, BlackBerry and Windows Phone. However, it would be interesting to see the numbers broken down by Specific Carriers.
CIRP (a.k.a Consumer Intelligence Research Partners) have conducted a survey from consumers who have activated their phones from the period of June to August 2012 to determine which OS dominates which Network. Well, they got some decent results, Verizon is the top-seller for Android phones while AT&T is dominated by iPhones. All other networks are inclined towards Android as well.
Carrier wise breakdown of OS looks something like this, 48% market share goes to Android, 28% claimed by iOS, 17% by Basic (Feature) phones, 4% BlackBerry and 1% Windows Phone.
There are reasons why Android and iOS dominate Verizon and AT&T, respectively. Verizon was the first to launch an Android phone, The Original Droid back in 2009 and since then it has grown from just one Android phone to numerous devices thus the #1 Android activations on the Network. While AT&T has been the longest iPhone seller for 5 years now and they have grown their iPhone subscribers list to become the number 1 iPhone sellers. Although, Both carriers now offer both Android devices and iPhones and it wouldn't take long to see Android dominating AT&T as well with it's range of devices in all types of budgets that makes it a much friendly OS, both technically and financially.
T-mobile and Sprint also account for the most Android devices sold. Although, seems like iPhones are catching up to Android on Sprint. Only time will tell which OS comes on top but till then Android is clearly and fairly the top Mobile OS in US and Worldwide.