This is something Apple should release! After you have jailbroken your iPhone 4 with the new Jailbreakme app you can download My3G from Cydia and be able to make FaceTime calls over the 3G network. The app is available on Cydia for $3.99 which is definitely worth it for what it offers.
Official description below:
Facetime over 3G!!!!
My3G: Take back your 3G! My3G makes Apps believe that they are on WiFi instead of 3G. With My3G, you can now watch HiDef YouTube videos, use Skype/Fring/Other VOIP apps over the 3G network!
– In-App popup request to enable/never ask for My3G to be enabled for that app
– In-App indicator when My3G changes the network from 3G to WiFi
– Ability to select which apps are My3G enabled and not.
– Default Apps – no configuration for the most popular apps!
– SBSettings Toggle! You can temporarily enable My3G for all apps or toggle back using your defined Apps list
– Dynamic enablement – Higher successs rate (near 100%) then 3G Unrestrictor (which does one off solutions for apps). My3G enables 3G indicators more dynamically.