The all new Facetime video chat feature is definitely one of the coolest improvements in the iPhone 4. According to a recent article on BusinessInsider.com, Facetime video calls won't use any of your on-air minutes. The video chat app requires you to be connected to a Wi-Fi network in order to work.
You'll be able to initiate a video call from within a voice call, but as soon as it's switches to Facetime,the voice call is dropped and Wi-Fi takes over. Apple is currently working with carriers to develop a method to use Facetime over 3G, but nothing has been announced yet."The voice call ends as soon as the FaceTime call connects," Apple tells us. "The FaceTime call is over Wi-Fi so does not use carrier minutes."
Source: Apple: FaceTime Video Calls Won't Use Your Carrier Minutes