There are over 17 million AT&T users that are grandfathered into the unlimited data plans. Last year AT&T claimed that 20,000% of their data traffic came from the top 5% of unlimited data users. Like us most of these users have unlimited data and use it religiously.
AT&T has officially announced their data throttling guidelines. Unlimited users on the HSPA+ will be throttled at 3GB and LTE users will be capped at 5GB with anything over resulting in a drastic reduction in data speeds. Of course the drop will still likely not be noticed from a users perspective unless downloading files since most videos will buffer before playing and not much requires more data than the throttled speeds.
Most unlimited users will never notice this cap because they still keep it under the limits although for those of you that stream movies and have rooted devices for tethering you may see a drop in data speeds the second half of your billing cycle. The data will return to normal after the billing cycle you go over.
UPDATE: While these limits have been enforced and seem to throttle our speeds once we hit their threshold its barely noticeable and we can still easily watch a Netflix movie and download anything else as fast as our home cable internet. Keep in mind that the throttled speeds are just there to improve overall data speeds for users across the board. I personally set my phone to notify me at 5GB of data so that I could test the speeds. It seems that after the throttle kicked in there was only about a 10% drop in my data speeds. Next I started downloading torrents to my phone through the data network instead of a computer to try and rack up some serious data usage. The outcome of that was my data use at around 16GB for the month and still my worst SpedTest was only about 17% from the peak speeds I normally see at the beginning of a billing cycle.