What began as a rumor yesterday has been officially confirmed: AT&T will begin throttling mobile broadband speeds later this year. In an announcement today, the carrier said it is taking steps to cope with the exploding demand for mobile data and the resulting network congestion. At least a part of that plan will include the reduction of data throughput for the remaining subscribers of the companies unlimited data plan -- at least those who exceed a certain monthly bandwidth threshold, anyway.

AT&T explained that the change will only affect 5% of unlimited subscribers who consume "extraordinary" levels of data. Such users account for much of the company's traffic, using 12 times more data than the average of all other smartphone customers. AT&T didn't say how much data the its heaviest users consume, but if it's any consolation, the company said you could receive thousands of emails, visit thousands of sites and stream hours of video each month without making the top 5%.

"Typically what puts someone in the top 5 percent is streaming very large amounts of video and music daily over the wireless network, not Wi-Fi. Streaming video apps, remote web camera apps, sending large data files (like video) and some online gaming are examples of applications that can use data quickly," the carrier explained. As you undoubtedly realize, Wi-Fi usage doesn't count against your mobile data consumption, and that includes the 26,000 public Wi-Fi hotspots AT&T offers.

It's unclear how much you'll be allowed to use before having your speeds reduced, nor has AT&T mentioned precisely how much they'll throttle data hogs. The change will go in effect starting October 1 and we assume the company will offer more details by then. It should be noted that customers on AT&T's tiered plans won't be affected. Subscribers of the company's $15 200MB DataPlus, $25 2GB DataPro or $45 4GB DataPro plans, you should still be able to pay for unthrottled overages.

This is yet another nail in the coffin of all-you-can-eat data plans. Besides Sprint, most major US carriers have implemented metered bandwidth of some form. AT&T introduced its tiered plans last summer and Verizon followed suit this month with capped plans and $10/GB overages. T-Mobile still offers unlimited consumption, but it does throttle speeds.