Just one more thing to think about... Mamiya now makes four new digital backs ranging from 22 MP to 56 MP. These back can be used on the RB-67 with an available adapter. See-->
http://www.mamiya.com/dm-series-digital-backs.html
It amazes me how such technically lame digital technology has managed to replace film so quickly, and amazes me even more how it has done so even in non-professional arenas, where none of the advantages of digital are even extremely advantageous.
I will be interested when there is a "full-frame"

D) 6x7 digital back that provides a
significantly larger file than what can be had from a small format digital SLR. Let us say, the same amount larger file over small format DSLR that 6x7 film has over 35mm film. Until then, forget it. There is no point for me. If I am not getting a file that is significantly larger, I would rather shoot with a high-end small format DSLR and enjoy all of its benefits over an RZ (cost and lens selection and speed being the most important).
8.64 sq-mm is the area of a full-frame small format DSLR sensor of about 22 megapixels. 42 sq-mm is the size of the theoretical full-frame 6x7 sensor. 4.86x larger than the small format DSLR at 22 megapixels. So, when they have a 106 megapixel full frame back for the RZ, and I am a professional photographer who could make money with such a gadget, I will be interested. Until then, I will shoot film through it.
Then...shall we even bother to go into dynamic range? 12 stops is absolutely pitiful at those prices. I would want 15 at the very least, as that is about the extreme of what I deal with on a regular basis using film.