Last month, AT&T decided to slow down data-transfer speeds to unlimited-data customers once they'd consumed three gigabytes of data in a month. This was an adjustment from a previous policy in which the carrier said the top 5 percent of data users with unlimited plans could be subject to throttling.

In Europe, the disparate treatment of users on different plans goes beyond offering better customer service to high-paying customers, Lynch says. There, when congestion forces a carrier to push users to another carrier's network, customers on unlimited data plans are the first to be pushed out.

"If I have a premium plan, [European carriers] want me on their network and to make sure I have the most robust, reliable experience," Lynch says. "I can identify in real-time—as the switching infrastructure is making decisions—who I want to be on and off network."

Since the carrier in the second network will offer the best service to its own customers, users that have been handed off from another carrier will bear the brunt of any congestion-related slowdowns. 

The telecom industry is working hard to sharpen the analytics tools its uses to tweak rates, and to refine service quality in specific high-traffic areas, says Chetan Sharma, a wireless analyst in Seattle. "They will use it for all sorts of purposes—they will try to monetize what can be understood from the big data," he says. "It's about how they want to treat congestion and how they want to treat customers who fall in different tiers and at different points in time."

In the United States, there is no indication that carriers give people with unlimited data plans subtly inferior treatment, except in imposing throttling, as the carriers already spell out in their policies.

Any such disparate treatment would not run afoul of FCC network neutrality regulations, which only apply to wired connections. And in any event, anecdotal reports about inferior service are hard to ascribe to any cause, because many factors can contribute to slower access or bumpy downloads.

As part of this trend, Nokia Siemens Networks launched a new tool last month at the Mobile World Congress in Barcelona that lets carriers mine and view information about network performance to find ways to change users' rates or improve service. For example, if a client used less data, the carrier could tell whether service in that neighborhood was poor and whether it would be profitable to improve it by installing another transmitter.

Of course, carriers also have troves of data on past network usage and Web access—information that will likely be used to sharpen targeted advertising in the future. "We're at the beginning of an era in big-data analytics," says Antonio Rodriguez, a venture capitalist who works at Matrix Partners in Cambridge, Massachusetts. "If you think about the treasure trove of data they have—it's question of how they tow the privacy line between what they have access to and what they do with it."