As our very own Sebastien correctly predicted, Sprint has announced that it is discontinuing its unlimited data plan. The only catch is that smartphones, including the iPhone, will still be able to take advantage of an all-you-can-eat data plan on Sprint’s network.

Sprint will begin imposing data caps on each customer’s next bill cycle, but iPhone users won’t be affected by the policy change. This move indicates that Sprint doesn’t plan on keeping its unlimited plan around in any form for very long.

Every other US carrier caps data usage, and T-Mobile throttles users that exceed a limit of 2GB each bill cycle.

Sprint’s new data policy applies to all of its “tablets, laptops, netbooks, USB and PC Card modems, and mobile hotspot devices.”

Macworld reports:

“While capping data use on its 3G network to 3GB, 5GB or 10GB per month, Sprint has continued to offer unlimited 4G service. Now, the cap for each of these plans will cover the subscriber’s combined monthly data use on both 3G and 4G. All data use above the cap will cost $0.05 per megabyte.

The monthly allowance for data use while roaming on non-Sprint networks will be 100MB for the 3GB plan and 300MB for the other two plans, with each megabyte over those caps charged at $0.25.”

The iPhone cannot take advantage of Sprint’s 4G network, and the carrier touts itself as having the best 4G coverage in the US.

Oddly enough, many customers have been complaining about slow data speeds on the Sprint iPhone. We’ve compared all three major US carriers’ data speeds on the iPhone 4S — Sprint was the slowest.

Due to the fact that the iPhone hasn’t been on Sprint’s network for even half a month, it’s obvious that the head honchos at Sprint wouldn’t be stupid enough to kill off unlimited data for new iPhone customers.. yet.

Sprint is still differentiating itself from AT&T and Verizon by offering the only remaining unlimited data plan in the US. We’ll see how long that lasts.