AT&T smartphone customers who are on the unlimited data plan may experience reduced speeds once their usage puts them in the top 5 percent of heaviest data users on the s network. Why? AT&T says smartphone customers with unlimited data plans use up to 12 times more data than the average of all other smartphone customers, thus straining the network. Says AT&T:

Typically what puts someone in the top 5 percent is streaming very large amounts of video and music daily over the wireless network, not Wi-Fi.  Streaming video apps, remote web camera apps, sending large data files (like video) and some online gaming are examples of applications that can use data quickly.

Before users’ bandwidth is throttled, AT&T says it will provide multiple notices, including a grace period. Customers on tiered plans won’t be affected because those who go over their limits pay for each additional megabyte and the company thinks that cost will impose a form of self-governance. Those users pay $25 for 2GB and $10 per gig thereafter. Whatever the options, we don't expect the heavy users to give their unlimited-data plan any time soon.

Source: AT&T