The NPD Group on Monday said it found that 30 percent of U.S. consumers who bought the iPhone 3G between June and August switched to AT&T, the exclusive operator for the iPhone, from another carrier. That's more than the average volume of phone users switching carriers, which is 23 percent, NPD said.

Verizon appears to have been hit the worst, according to NPD. Of new AT&T iPhone customers during that period, 47 percent came from Verizon, NPD said. Just 19 percent switched from Sprint and 24 percent came from T-Mobile.

According to NPD, the iPhone 3G was the number-one smartphone based on unit sales from June to August. The BlackBerry Curve was number two, followed by the BlackBerry Pearl.

The iPhone has also driven down smartphone prices, NPD said. The average price of a smartphone sold between June and August was US$174, down from $236 during the same time last year, NPD said. The G1 costs $179 with a contract.