AT&T's restrictions on cell data usage have evolved over time. Recently, download data caps have been eased, from 10 MB to 20 MB. And since October 2009, VoIP apps have been allowed to use AT&T's data network to place calls.
The elephant in the airwaves, however, has been Skype. Several months after AT&T allowed VoIP apps to use cell data, Skype was still without an update. The company, via a blog posting on its site, stated that it wanted to take the time to ensure that audio fidelity was at the best quality.
I made several calls over AT&T's 3G network this afternoon using the updated app and was surprised at how good the quality was. Each person I spoke with said they could barely tell the difference between this and a call using AT&T's voice network.