I was initially running iOS 6 Beta on my iPhone 4S and Facetime worked well over cellular. I am now running the real iOS 6 on it and the last time I tried making a Facetime Call over Cellular it still worked. So even though we are under AT&T's network it doesn't prevent us from using FaceTime
So you're saying Facetime works with Straight Talk? It must not take long before you get to ST's infamous 2GB unlimited cutoff.