Well, I'm not sure it would work, but there is a tweak called "3G Unrestrictor" on Cydia. It tricks apps into thinking they are on WiFi. Has worked/does work for me.
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Heh. I was typing this and went to check Cydia to see if it's free or paid.
First, yes, it's a paid app ($3.99 USD) but - there's a big [new] notice on the page: "Now with FaceTime over 3G support!"
So, I'd say yes, after you buy it, FaceTime should be fine over 3G.
Marilyn