If you're on an unlimited data plan on AT&T, and consider yourself a heavy data user, you may want to pay attention. Like other carriers, AT&T is going to begin throttling speeds once a certain threshold is reached. However, unlike other carriers, AT&T has decided instead of setting a fixed number as the limit, they will simply reduce speeds for customers falling into the top 5% of the heaviest data users. Users will still have unlimited data, and speeds will be restored at the start of the next billing cycle.
The amount of data usage of our top 5 percent of heaviest users varies from month to month, based on the usage of others and the ever-increasing demand for mobile broadband services. To rank among the top 5 percent, you have to use an extraordinary amount of data in a single billing period.
There will be no changes for the vast majority of customers. It's not how much time you spend using your device, it's what you do with it. You can send or receive thousands of emails, surf thousands of Web pages and watch hours of streaming video every month and not be in the top 5 percent of data users.
Chances are, this change won't affect you, unless you use an extraordinary amount of data through streaming tons of video/music daily, streaming remote web camera apps, sending large data files, online gaming, or tethering. If you utilize wifi for these things, you aren't using your data plan, so therefore you won't be affected. The changes go into effect October 1, 2011. You can find full details at the source link below.