One of the new features of iOS 6 is to make FaceTime call via 3G or 4G networks. So far this feature was not allowed by Apple, most likely because carriers were not really happy to have users draining such bandwidth resources from their unlimited data subscription. It seems that AT&T and other carrier have decided to have FaceTime subscription-based option:
As indicated by this capture, FaceTime via 4G network will invite the user to activate this feature, and to subscribe to an additional option. Of course, this might vary from one country to the other, but it is unlikely that many carriers will allow FaceTime call for free considering the bandwidth consumption.