If you've ever needed a reason to jailbreak your iPhone 4, this could be it. Thanks to a jailbreak application called My3G, you can have FaceTime calls over 3G without the need to connect your device to Wi-Fi. The application will cost you some money, but the price of jailbreaking, and the price of the application, may be worth it for some.
The application basically takes your 3G connection and "converts" it so that your iPhone 4 thinks it is on a Wi-Fi network. And as you can see from the video below (via 9to5 Mac), the connection works very well with a FaceTime call.
To get the My3G application, you must download the "Rock store" from Cydia once you've jailbroken your iPhone 4. You can get get My3G from the Rock store for $3.99 US. Cydia also has the same version of the application for $2.79, a slightly cheaper price.