U.S. wireless smartphone subscribers will do almost anything to keep their unlimited data plans, which could spell trouble down the road for AT&T, the only U.S. operator without an unlimited plan.

For the growing number of smartphone users in the U.S. market, loyalty to a particular phone or a specific carrier is trumped by the availability of an unlimited data plan. Cell phone consumers are a notoriously fickle bunch. And for the past couple of years, wireless operators have been stepping over each other to gain exclusive deals on hot new phones to attract new subscribers.

This strategy has helped operators, such as AT&T, win new subscribers. But a recent survey by Wall Street research firm Sanford Bernstein indicates that for existing smartphone customers, the security of knowing they won't exceed a monthly data usage cap takes priority over which carrier's network they subscribe to or which phone they use.

"It is fashionable to argue that loyalty to carriers is dead (except perhaps to Verizon Wireless, whose service level is perceived to be markedly higher than that of its competitors)," writes Craig Moffett, an equities analyst with Sanford Bernstein who wrote the report analyzing the study. "The new conventional wisdom is that carrier loyalty has been replaced with loyalty to the device. But high inclination to switch carriers and phones to maintain an unlimited plan suggest that perhaps the plan itself is more important than either one."

What's this mean for wireless operators For AT&T, which has benefited from die-hard loyalty to the Apple iPhone for more than three years, it means the carrier may be vulnerable when it loses the exclusive contract for the iPhone. As for the other three wireless operators that have not yet eliminated an unlimited data plan, it means they might be able to scoop up valuable customers.

AT&T was the first major U.S. carrier to eliminate its $30 a month unlimited data plan. In June, right before it launched the iPhone 4, the company adopted a two-tiered pricing model for new subscribers. The new pricing scheme forces new smartphone users to sign up for either a $25 a month 2GB monthly plan or a 200MB $15 a month plan.

AT&T "grandfathered" existing smartphone subscribers, allowing them to keep their $30 unlimited data plans. But according to Sanford Bernstein's survey, the move has left a bitter taste in some AT&T subscribers' mouths.

About a third of the more than 800 people responding to the Sanford Bernstein survey said AT&T's move toward usage-based billing sparked negative sentiment toward the company. The study also suggests that if forced to take a tiered data plan in lieu of an unlimited plan, a large proportion of consumers would switch carriers even if it means buying a new phone and paying a premium on a different carrier for the unlimited plan.

AT&T spokesman Mark Siegel said that AT&T's new pricing plans give customers more flexibility and more choice when it comes to data.

"We have found that our customers in fact like usage-based billing," he said. "They appreciate having choices in data plans. This is probably because a majority of customers can reduce their costs through our plans."

He said the response to the tiered plans has been strong both with new customers and existing customers. AT&T has been signing up huge numbers of new smartphone users. In the third quarter, it activated 8 million smartphone accounts, of which 5.2 million of those devices were iPhones. This was the largest number of iPhone activations for AT&T in a single quarter.

But Siegel would not disclose how many of the existing iPhone subscribers have actually switched to one of the tiered plans. Moffett speculates in his report that the loyalty consumers feel toward the iPhone is uncharacteristic. Consumers don't get the iPhone because they want to be on AT&T's network' they subscribe to AT&T because it's the only place in the U.S. to get the iPhone. Once the iPhone is available on other networks, AT&T will likely find itself in the same position as Sprint Nextel and T-Mobile USA, whose customers appear to have far less loyalty to the network than they do the cost of the overall plan.

"Respondents were more loyal to their pricing plan than to either their carrier or their phone," he says in the report. "More cynically, they were simply not loyal, and cared merely about price."

Copycats The big question since AT&T introduced its plan is whether its competitors would follow with usage based plans of their own. So far only Verizon Wireless has dipped its toe in the waters of tiered data pricing. Verizon recently introduced a new tier of service that offers 150 MB of data for $15 a month. But the company wisely kept its $30 a monthly unlimited plan.

Since the plan was introduced, executives have been quick to point out that the tiered offer is for the company's 3G pricing. It will be launching its 4G LTE network later this year, and when it does, it will adopt a new pricing plan. But according to a recent interview with The Wall Street Journal, even though Verizon will likely offer a tiered service offering, it may also keep the unlimited plan.

Meanwhile, Sprint Nextel and T-Mobile, which are each rolling out upgrades to their network and each with far fewer customers than either AT&T or Verizon Wireless, say they may consider usage based pricing in the future. But for now, they will continue offering unlimited data service.

This could prove to be a major differentiator for these operators, according to Moffett.

"Unlimited data plans could become a major source of differentiation, attracting customers and giving still-unlimited carriers a subscribership boost," he writes. "At the same time, however, carriers who maintain unlimited pricing in the face of AT&T's move to usage based pricing (and Verizon's expected move to follow) could self-select to the heaviest users, impairing future profitability."

The problem that wireless carriers face right now is that they need to increase adoption of data services without increasing consumption of those services. They're unable to keep pace with network upgrades to meet data demands. But more importantly, they can't afford these upgrades. Flat-rate unlimited pricing reduces how much revenue carriers make on each bit of data traversing their networks.

In other words, the more data consumers use on an unlimited data plan, the less money carriers make from that service on a per-bit basis. So they need to be able to find a balance in which they can charge customers who use more of the network's resources more money for access to those resources.

Of course, this is very unpopular with consumers, who have gotten used to all-you-can-eat Internet data buffets.

People are willing to overpay For most consumers, who are likely only consuming about 300MB of data per month, the $25 plan from AT&T would actually save them money since it's very unlikely they'd ever be charged overages for exceeding the 2GB cap.

But many people are willing to pay for more than they need simply to have the peace of mind that they won't be hit with an overage charge. Even though the math doesn't work in their favor, the psychology of paying extra for this insurance is worth it. Indeed, the Sanford Bernstein survey found that a large number of light users, people consuming less than 200MB of data per month, prefer unlimited plans, too, even if their usage patterns suggest that they would save money by getting a plan that's capped. The problem is that people drastically overestimate how much time they spend doing data-intensive activities and thus they grossly exaggerate how much data they use.

Another study by the Technology Policy Institute (PDF) that was published last week on the cost of broadband plans, discovered that most consumers on broadband data plans with data caps actually save between 15 percent and 25 percent compared with people who are on unlimited plans. The report, authored by Scott Wallsten and James Riso, stated that residential standalone broadband plans with bit caps are, on average, $164 less per year than similar, but unlimited plans.

"Policymakers should not immediately conclude that data caps and other pricing schemes that differ from traditional unlimited plans are necessarily bad," the report said. Instead, the authors suggest, pricing trends should be evaluated over time to identify the effects on prices, investment and usage.

AT&T's Siegel said the company's less expensive data plans have helped AT&T tap a new crop of customers. The Sanford Bernstein survey also suggests that capped services, priced lower than unlimited plans can expand the market for operators.

"There is some upside for AT&T and other UBP (usage based pricing) carriers, in the form of late smartphone adopters," Moffett writes in the report. "There are people currently without smartphones but who would be interested in adopting one in light of cheaper data plans."

The biggest hurdle may be simply persuading consumers to abandon the safety net of unlimited plans. Until then, Sprint, T-Mobile and Verizon Wireless may rack up some new customers at AT&T's expense.