Rumors indicate that it will open in time for Christmas 2010 which makes a lot of sense. Stay tuned as more information arises.
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! Features: - 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.