MICGadget, published some images of iPhone 4 64 GB, internal test units. Some of the test units are available for sale in the Hong Kong black market, that clearly seem to prototypes of a 64 GB iPhone 4. You can see in the attached image that displays the characteristics of the terminal, although many consider it a gross manipulation.
The double X on the back of the phone said that this is an irresolvable internal circulation model of which perhaps will never see the light. Who is selling them ensures that you have very few units (normal, having stolen in between). The phone also model number and FCC ID camouflaged with several X and is not blocked.