Back in July AT&T said that it will "Throttle Top 5% of Unlimited Subscribers" (here). Said and done.
Ed Sutherland reports to Cultofmac that the operator is sending SMS messages to some subscribers and the text reads: “Your data usage is among the top 5 percent of users. Data speeds for the rest of your current bill cycle may be reduced.”
AT&T's dataplans web pages explains (here): "One new measure is to reduce the data throughput speed experienced by a small minority of smartphone customers who are still on unlimited data plans. These customers represent only five percent of our smartphone data customers but they use an extraordinary amount of data each month. On average, they use about 12 times more data than the average of all other smartphone data customers".
To avoid that, AT&T recommends users to use the free Wi-Fi services or "If speed is of top importance, you may wish to switch to a tiered usage data plan. Tiered data plan customers can pay for more data when they need it and will not see reduced speeds" (here) .
See also "AT&T Continues to Push UBB - Now to Illegal Tethering" - here and "Who Shapes Traffic in the US?" - here.