When Apple announced that they were enabling Facetime over cellular connections with iOS 6, I was waiting for the other shoe to drop. You see, FaceTime uses a ton of data, and I expected customers to revolt once they used all the data in their "unlimited" data plans or generated massive overage charges. I also wouldn't have been particularly surprised if AT&T blocked Facetime use over cellular for the grandfathered-in "unlimited" plans that they've been trying to kill for a couple of years now. But no, AT&T decided to do us all a big favor, by limiting Facetime over cellular to people who have signed up for the ridiculously-overpriced shared data plan, MobileShare.
Let's be perfectly frank, unfettered access to a data-hogging video conference service is a massive revenue-generation opportunity for the company, and they're putting that aside to help us all out.
The problem is that Facetime is a data hog, even compared to services that stream music or movies. You see, Facetime runs two complete video streams simultaneously: one up, and one down. To test this, I connected an iPhone and iPad to a router that lets me monitor live bandwidth usage and made a series of Facetime calls to people outside my network. In my tests, Facetime actually used between 300Kbps and 900Kbps per video stream, depending on the amount of motion in the video streams. Mulitply that by two for inbound and outbound streams, and that means a Facetime call uses a minimum of 600kbps and a maximum of 1.8Mbps. Doing the math, a 20 minute call will use a minimum of 88MB of your data plan and a maximum of 264MB. Consider that typical data plans for the iPhone include 3GB of data/month, and you can blow through an entire plan worth of data in a few hours of Facetiming. Once you've used all the data in your plan, Facetime calls would cost about $10/hour.
There is one big caveat--Apple could reduce the bitrate of Facetime over cellular connections. We aren't able to test that since iOS 6 beta users are prohibited from Facetiming over AT&T's network unless they have the correct data plan.
While you may be pissed off that AT&T is prohibiting its users from using FaceTime over its network, they're sacrificing millions of dollars in potential overage charges to save you, the mobile broadband customer, from costly charges. Of course, if you really, really want Facetime over cellular, they will make that service available to customers who opt into the costliest data plans they offer.