Many iPhone users are willing to switch to AT&T in order to use the device, judging by the 40% of iPhone activations the carrier saw during the December quarter coming from new customers. In its 4th quarter earnings announcement, AT&T said there were some 1.9 million iPhone activations during the quarter, and that iPhone 3G customers were worth more than other customers.

AT&T is the exclusive iPhone carrier for the U.S., though some iPhone users have hacked their iPhones to work on other carriers. The company heavily subsidizes sales of the iPhone, possibly as much as $400 per iPhone, according to some reports.

The reason why any carrier would be willing to do so, of course, is the possibility of luring new customers to the company's voice and data plans, the end-all, be-all goal of carriers throughout the world. During the December quarter, AT&T apparently achieved this goal to the tune of 760,000 new customers in a tough economic climate, thanks to the iPhone.

AT&T also took pains to make it clear to shareholders that getting those iPhone customers was worth the effort, saying, "iPhone 3G continues to deliver high-value subscribers with significantly higher ARPU (average monthly revenues per subscriber) and lower churn than AT&T's postpaid subscriber average."

That means that iPhone customers are willing to pay more than other customers when using their phones, and that they are less likely to drop AT&T and move to another carrier.

Shares in AT&T traded higher Wednesday, closing at US$25.37 per share, a gain of $0.10 (+0.40%). Apple ended the day at $94.20 per share, a gain of $3.47 (+3.82%).