Currently, on the iPhone 4 and 4S, FaceTime only works over Wi-Fi (yes, there are some workarounds to get it to work over 3G, but Apple doesn't officially support it). In iOS 5, doing FaceTime over a cellular data network is now part of the settings menu but it looks like the carriers have control over whether it's enabled.
Chances of implementation: 50 percent
Photo by: Apple
Caption by: David Carnoy, CNET