AT&T Mobile Share will launch August 23rd and offers users free talk, text and various amount of data per month to cover all of their devices. 1GB will cost you $40 plus $45 for each smartphone on the plan while 20GB runs $200 plus $30 for each smartphone or feature phone. Tablets cost $10 each to be added to a Moble Share Plan. There has been speculation that AT&T would restrict FaceTime over Cellular since evidence to that effect was found in the beta version of iOS 6.
As far as the other major carriers in the U.S. that carry the Apple iPhone, Sprint says that it will not charge for FaceTime over Cellular while Verizon has yet to say if it will have restrictions on the feature.
source: MacRumors via IntoMobile