According to AT&T, they will give prior warnings before throttling:
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.
Reading your comments, is this not happening? I'm not a heavy user of data, and I try to use WIFI whenever possible, but a few times I have gotten really close to 2 GBs of data. I just want to make sure, I'll know before I'm throttled, right?