Can you FaceTime over 3G with jailbroken iPhone 4?
Yes to all of your questions.
The Jailbreak app My3G tricks your iPhone into thinking it is on WiFi, enabling you to use FaceTime over 3G.
Source: http://www.macrumors.com/2010/08/02/facetime-over-3g-comes-to-jailbroken-iphone-4s/
Just to update an old question, this is now a carrier issue - most carriers either allow Facetime over cellular or have an option to pay for it.