I guess that AT&T learned from its "mistakes". Users that are a part of its grandfathered unlimited data plan will not truly receive unlimited data. AT&T has announced some changes (or a clarification, as it puts it) to its throttling procedures today that will at least give you a bit more room to work with. For customers on an unlimited plan with a 3G or "4G" phone (i.e. HSPA+), you'll now be able to enjoy full data speeds up to 3GB, after which you'll then see your speeds decrease until the start of the next billing cycle. If you have a 4G LTE phone, however, you'll have a full 5GB to play with before the throttling kicks in.
That's as opposed to the roughly 2GB of full data speeds that was available in both cases before.