As planned, AT&T has started to throttle users on the unlimited plans who place in the top 5% of data usage for that month.
Although originally planned to start October 1, many users are reporting that their data usage is already being throttled. No word on the exact speeds yet.
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.
Verizon has too been throttling data for unlimited customers starting a few weeks back.
Remember that this only applies to unlimited data customers using a smartphone. Users reported that they received the message on throttling at or around 10GB.