Apple’s hot iPhone 3G is now the most popular mobile phone in the U.S., according to market researcher NPD.

iPhone 3G pipped Motorola’s RAZR (the top-seller for the past 12 quarters) to take the No-1 spot in the third quarter.

Since the iPhone debuted about 17 months ago, sales of the smartphone have been on a tear. In the last quarter alone (ended September 27, 2008), Apple sold 6.892 million iPhones compared to 1.119 million units in the same quarter a year ago.

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Here are the top-selling handsets (by rank) in the U.S. based on unit sales:

1. iPhone 3G
2. RAZR V3 (all models)
3. RIM Blackberry Curve (all models)
4. LG Rumor
5. LG enV2

We think the upcoming touchscreen-based Blackberry Storm could have some impact on iPhone sales in the coming quarters.

Despite strong consumer sales of the iPhone 3G and and the usual seasonal boost after Q2, domestic handset purchases declined 15% year over year in Q3 to 32 million units.

NPD said consumer handset sales revenue fell 10% to $2.9 billion although the average selling price rose 6% to $88 (presumably because more smartphones like the $199 or $299 iPhones are being purchased these days).