AT&T will begin limiting data speeds for the top 5 percent of wireless data users who currently have unlimited data plans, the company announced Friday. Many AT&T customers currently operate under a tiered data plan, but customers who had unlimited usage before the tiered plans were initiated about a year ago were able to continue downloading and uploading an unlimited amount of data over the company's wireless network each month. Starting Oct. 1, however, AT&T will start dialing down the speed on its heaviest users.