I am using 3G unrestrictor on iPhone 4 OS 4.1 and facetime is working over 3G. You have to turn on the "Enable IsDirect flag" to make it work. You can do this within the 3G Unrestrictor App by hitting the ">" arrow.
i just tried that.. but it didn't work .. it would ring if someone calls.. but when i hit the answer key.. it say's connecting but then cuts off and ask me to connect to a wifi network
try turn on wifi but do not connect to any network, it worked for my friend, tested it out with him today, but its laggy tho