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.
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.
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.