Bernard, you missed the best part of the rumor, namely that the new MX camera would be a rangefinder similar in size to the Mamiya 6, with a new mount for the MX lenses, and an adapter for Nikon FX and DX lenses. Use of FX and DX lenses, via the adapter, would result in an in-camera crop, similar to DX on the D3.
Indeed, I missed that part. I would be surprised if it were a rangefinder, but you never know.
As far as believing or not... you have to wonder. What could come next after the current crop of 20+ MP FF bodies that brings actual value to photographers while allowing camera manufacturers to keep making big money?
The current lenses are becoming the limiting factor on 35 mm.
Nikon has publicly declared that their intention is to focus on high end and smaller volumes.
So all in all, it would make sense IMHO for them to try to expand the existing categories and find real ways to deliver better values for photographers looking for even better image quality without spending 40.000+ US$ for a full system.
People have been willing to pay 8.000 US$ for a 1ds3/ZD, there is little doubt that at least the same amount of people would be willing to pay 10.000 US$ for a medium format body with all the advantages of a Nikon make.
It is at least plausible enough to consider that Nikon might think of doing such things. They have made very bold moves before, the D1 started the DSLR revolution and cut the price of the super high end Kodak in 4 for instance,... would a Nikon MX body be that different?