One of the largest mobile Web ad networks, AdMob, says that Apple’s iPhones now dominate the U.S. mobile Web traffic, gives the iPhone a majority 51 percent share of smartphone market in the country.

Based on analysis of the billions of ad requests in January conducted by AdMob, here is the iPhone’s achievement:

• The iPhone OS now represents 51% of U.S. smartphone traffic, leaving RIM’s BlackBerry (19%) and Microsoft’s Windows Mobile (14%) in the dust.
• The iPhone and iPod touch now represent 18.3% of worldwide ad traffic — second only to Nokia with 30.1%.
• Worldwide requests from Apple devices grew 28% month over month to 1.2 billion in January.
• The iPod touch is rapidly catching up to the iPhone; it now represents 40% of Apple requests, up from 20% in September.

[Fortune, Apple Insider]