Really? How did you manage that? Mine still redirects me to the AT&T site to change to an "eligible" plan.
I just received my iPad mini, logged into my AT&T grandfathered unlimited data plan, and activated FaceTime. Oddly enough, the Use Over Cellular was already marked as ON. I tested it, and it worked.
I also tested an already active iPhone 5 with AT&T grandfathered unlimited data. It didn't work initially, but after restarting the iPhone 5 once, it automatically switched ON Use Cellular Data.
iPhone 4S with AT&T grandfathered unlimited data running iOS 5.X still doesn't have FaceTime over cellular. I don't plan to update to iOS 6 to try to get FaceTime over cellular to work, but if FaceTime over cellular eventually works with iOS 5, I'll update here.