According to NPD's figures, Motorola's RAZR v3 (all models) fell to second place during the quarter, followed by Research In Motion's Blackberry Curve (all models), the LG Rumor and the LG enV2.

Apple has sold more than 7 million iPhone 3Gs since launching the device in July, but the iPhone's success comes at a time of sluggish overall handset sales in the U.S. adult consumer segment. According to NPD, handset sales in the category dropped 15 percent year-on-year during the quarter to 32 million units.

The RAZR has had a good run, but the fact that it lacks a QWERTY keyboard could be contributing to its waning popularity. Mobile phones with QWERTY keyboards accounted for 30 percent of handset sales in Q3, compared to just 11 percent in the same quarter the previous year, NPD noted.

The iPhone has cornered the market in terms of coolness, but according to a study released last week, it's also far more dependable than the BlackBerry and Palm Treo.

SquareTrade, which sells extended mobile device warranties, collected data from 15,000 mobile devices and found that in the first year of ownership, the iPhone had a malfunction rate of 5.6 percent, compared to 11.2 percent for the BlackBerry and 16.2 percent for the Treo.

But despite iPhone's third-quarter success, the prospect of weakening consumer demand brought on by the nose-diving economy could be prompting Apple to scale back production in the fourth quarter.

Last week, FBR Capital Markets analyst Craig Berger said Apple may cut iPhone production by more than 40 percent in the calendar fourth quarter compared to the third quarter, and UBS analyst Maynard Um predicted that Apple will ship around 2 million fewer iPhones in the fourth quarter than it did in the previous one.