I have yet to get this on my iPhone 4S with AT&T. I thought they were giving it to people with unlimited data plans, yet I keep getting the warning telling me I need the Shared data plan to get FaceTime over 4G.
Oh, fellow romanian, you can try a usefull app called 3G Unrestricor in the Cydia Store that let's you FaceTime over Cellular and many other things like iCloud back-up & PhotoStream upload over Cellular, unlimited app size downloads and so on.
Hai noroc ! (english : cheers )
( I have just restored my iPhone 4 and now the option is gone.Something is wrong,I facetimed my brother today over 3G and it worked,maybe it was the jailbreak or siriport,I don't know...but it worked pretty good. Salut tuturor romanilor!!!!