Does iPhone 4 FaceTime video calling use cellular voice plan minutes?
No. Even if you initiate a video chat during a phone call, the iPhone will automatically switch to Wi-Fi from that point forward and your cellular plan will not be charged voice minutes.
Apple has confirmed that FaceTime video chat will only be available via Wi-Fi through the end of 2010. The company is discussing the future of video calling technology and billing with cellular carriers.
In the meantime, you can make video calls on Wi-Fi and they will not be charged to your voice minutes.