FaceTime billing
Video calling has existed in Europe and elsewhere for a long time now. Networks usually charge separately for it by the minute (i.e. it’s billed like a call, but at a higher rate due to the additional bandwidth required). This would also make sense for AT&T given that they’ll want to keep video traffic to a reasonable level to avoid crippling their 3G network, and voice calling would quickly burn through your monthly data allowance.
I’d imagine that carriers will bundle a certain number of FaceTime minutes with each package, and then you’d have to pay for extra minutes on top. This isn’t such a bad option given that you’ll also be able to place Wi-Fi video calls for free.
With a bit of lack, Apple might even add some extra bells and whistles like video mail, greeting message and iChat/AIM support at the same time. 🙂