The guys at LaptopMag tested out a FaceTime call over 3G by using the HTC Evo's Mobile Hotspot feature, with its 4G turned off. The Evo used its 3G network to broadcast a Wi-Fi signal to get the iPhone 4 to connect to. Sneaky. The other iPhone 4 was connected to a real Wi-Fi network and if things went swimmingly, it'd be another instance of AT&T handcuffing the iPhone. But swim it did not, their video test went horribly wrong. According to them, "audio came through only in patches, and video was like a slideshow at best".
That's not to say that FaceTime over 3G is always going to look like this. But since your 3G connection is volatile and hinges on so many different things like signal strength, tower proximity, how many users are connected to a tower, being indoors or outdoors, magnetism, unicorns, fairy dust and how you hold your phone, you just never know what you're going to get. Over Wi-Fi, it's consistent. Over 3G, it's a crapshoot. [LaptopMag]