From the AT&T press release:

AT&T is continuing to simplify pricing with new plans that eliminate data overage charges for our wireless customers.

The new Mobile Share Advantage plans are available Aug. 21. Consumers and businesses alike can gain a wireless experience, without overage charges. Instead of overage charges, after customers use all of their high-speed data amounts, all data usage will be reduced to a maximum of 128 kbps for the rest of their bill cycle.

In effect, once you hit your data cap, your data speed will slow to a relative crawl.

From the fine print:

No charge for overage. After all your high-speed data allotments are used, all data usage is slowed to a max of 128 Kbps (2G speed) for the rest of the bill cycle. You will have basic data use for viewing a web page or checking email. Audio and video streaming, picture and video messaging, select apps and other services, as well as other usage, including sponsored data, will be impacted and may not be fully functional.

I’m assuming you can pay to add more high-speed data for the month, so this seems like a positive development.