Research firm ChangeWave has done an analysis that shows the iPhone to be a critical factor in AT&T's success and in its ongoing battle with Verizon. Absent the iPhone AT&T would probably not be holding up quite as well. Below are some charts supporting the thesis. Look at the higher future intent to switch to AT&T in the first chart compared with lower customer satisfaction for AT&T in the second (vs. Verizon): 

Part of the explanation may reside in the difference between populations answering the questions (existing customers who have experience with the carrier vs non-customers who do not). But it's got to be about the iPhone.

In contrast to AT&T's satisfaction numbers, among so-called smartphones the iPhone is highest rated according to JD Power & Associates. Here's what ChangeWave has to say:

Note that dating back to the period just before the original Apple (AAPL) iPhone rollout, AT&T has managed to stay in first place in terms of future demand.

A coincidence? Not likely.

But the big question going forward is how long can AT&T keep its exclusivity agreement with Apple and ride the iPhone’s coattails? While the current deal is set to expire next year, AT&T is in talks with Apple to extend it until 2011.

Apple has affirmed its relationship with AT&T publicly. However there are also suggestions promoted by Verizon itself that the iPhone or an Apple handset might make its way to the carrier in the next year or so. 

Verizon is fighting back with the BlackBerry Storm (and within six months the Pre).