Apple’s iPhone 3G dethroned the long reigning king of the handsets, the Motorola RAZR, as the leading handset purchased by adult customers in the U.S. during the third quarter of the year, according to market research firm NPD, which is a huge victory for the smartphone in its battle for mainstream penetration.

The news puts an end to the RAZR’s huge three-year run as king of the marketplace, with the Motorola handset falling to the second slot, followed by the Blackberry Curve, then the LG Rumor.

Despite stronger sales of the iPhone, the mobile phone segment failed to see its normal seasonal jump following the end of the second quarter, with US sales to adult consumers declining 15 percent year over year in the third quarter to 32 million units. Similarly, consumer handset sales revenue fell 10 percent to $2.9 billion, even as the average selling price (ASP) rose 6 percent to $88. Much of this can be attributed to the economic climate in the US, which is downright lousy at the moment.

Word of the iPhone’s rising popularity amongst average consumers comes just weeks after Apple announced that it sold more handsets in the third quarter than its chief competitor RIM, making it the second largest smartphone manufacturer in the world by volume. Huge numbers, and still growing.