As reported on Thursday by 9to5Mac, AT&T on Friday confirmed plans to throttle the data transfer speeds of smartphone users still on “unlimited” data plans. [ad#Google Adsense 300×250 in story]

“Starting October 1, smartphone customers with unlimited data plans may experience reduced speeds once their usage in a billing cycle reaches the level that puts them among the top 5 percent of heaviest data users.  These customers can still use unlimited data and their speeds will be restored with the start of the next billing cycle.  Before you are affected, we will provide multiple notices, including a grace period.”

AT&T says that such users transfer 12 times more data than the average user does. The company also reminds users that switching to Wi-Fi – even Wi-Fi hosted by an AT&T-branded hotspot – does not incur additional load on the cellular data network. The company encourages users of Wi-Fi enabled smartphones (such as the iPhone) to use Wi-Fi wherever they can to help reduce network congestion.

AT&T also used the service notice as an opportunity to plug their planned merger with T-Mobile.

“Nothing short of completing the T-Mobile merger will provide additional spectrum capacity to address these near term challenges.”