AT&T announced that it will reduce the data throughput speed of its biggest data users on the unlimited data plan. The company stated it's responding to the explosive growth in data usage and the network congestion that comes with it. 

AT&T stressed that the move will not affect the vast majority of its customers, who are either on tiered plans or are moderate data users on the unlimited plan. 

Instead, it's targeting the top 5 percent of heaviest data users on the unlimited plan, who use 12 times more data than other users, according to AT&T.

The company said it would start metering once a user hits a certain threshold in each billing cycle that puts them at the top 5 percent of data usage.

Currently, Verizon and T-Mobile do not offer true unlimited data plans either.  Sprint, therefore, is the only major carrier to still offer true unlimited data plans.

Throttling the heaviest users makes sense on many levels; it doesn't affect the vast majority of users and frees up a disproportionate amount of capacity. 

However, there are still major advantages to offering an unlimited plan, said Sprint CEO Dan Hesse in an Engadget interview published July 19.

Hesse said people like the simplicity of an unlimited plan.  With Sprint's major rivals getting out of the unlimited business, the appeal may even be more pronounced.

However, Sprint's unique offering may also attract many of the heaviest data users.  When asked by Engadget what Sprint would do in that case, Hesse said Sprint is working with OEMs and app developers to implement solutions like WiFi offload.

Sprint could also raise its prices for unlimited data plans, as it did in January 2011. 

However, "nothing is guaranteed forever," said Hesse.

Engadget reporters said they get the impression that Sprint's "transition [to a tiered route] is imminent - even if it's still a few years out."