About 30 percent of first time U.S. iPhone 3G buyers between June and August switched to AT&T from other carriers based on data from NPD Group. Apple's combination iPod and smartphone is selling well enough, in fact, to have moved in behind Motorola's RAZR V3 as the second best selling mobile phone in the U.S., according to MarketWatch.

About 47 percent of those switchers were abandoning Verizon Wireless, with about 24 percent coming from T-Mobile, and another 19 percent from Sprint.

The iPhone 3G has helped Apple increase its slice of the smartphone market, too. NPD data showed that the first generation iPhone held about 11 percent of the consumer market for smartphones from January through May 2008, but that number climbed to 17 percent by August. Ross Rubin, NPD Group director of industry analysis, commented "The launch of the lower-priced iPhone 3G was a boon to overall consumer smartphone sales."

Apple's second generation iPhone is proving popular in the smartphone segment, too. The iPhone 3G managed to come in as the most popular selling smartphone in the U.S. from June through August. RIM's Blackberry Curve and Pearl came in at number two and three respectively, and the Palm Centro was the fourth most popular smartphone.

When Apple first launched the iPhone, many industry experts weren't expecting it to sell as well as it did, and now that the iPhone 3G is proving to be even more popular, many of those experts are now scrambling to try to catch up. As Apple continues to strengthen its position in the cell phone market, that may be harder to do.