There are already many jailbreak apps out there that let you use FaceTime over 3G. Here’s yet another one called FaceBreak which lets you do that. All you need to do is install it from Cydia and respring or reboot your iPhone 4 and you’re good to go.
FaceTime can be enabled or disabled from the Settings App as well.
FaceTime costs $1.49. You need to have a jailbroken iPhone 4 to install FaceTime. See JailbreakMe on how to do that.
FaceTime isn’t as full featured as My3G but it’s cheaper. My3G costs $3.99. Read more about My3G.