That's the conclusion one draws, anyhow, after getting a glimpse at the latest quarterly reports for U.S. wireless companies. Sprint was the latest to tout its iPhone success, revealing that it had activated 1.5 million iPhones when it on Wednesday.

Sprint's announcement follows earlier reports from AT&T and Verizon, the other national iPhone carriers in the U.S. On Monday, during the first quarter. The week before, as part of a conference call to discuss its , Verizon said it activated 3.2 million iPhones during the first three months of 2012.

Both AT&T and Verizon saw increases in the number of iPhones activations from the year-ago quarter. In the first three months of 2011, AT&T activated 3.6 million iPhones while Verizon activated 2.2 million in its first quarter as an iPhone carrier.

Impressive as those tallies are, digging a little deeper into the numbers reveals how important the iPhone is to each of the three carriers.

The popularity of Apple's phone does come with a cost for carriers. Each of the companies subsidizes the cost of the iPhone, taking a short-term loss on the cost of the device in exchange for profits over the course of a typical two-year service contract. Sprint reported Wednesday that its subsidy fees had increased by a half-billion dollars over a year ago--from $1.1 billion to $1.6 billion--thanks mostly to iPhone sales. The device, the company said in its earnings report, "on average carries a higher subsidy rate per handset as compared to other handsets."