The persistent rumor of a medium format Nikon seems to make less and less sense as the prices of other MF systems drop. With the entry to all the other systems at $25,000 - $30,000, Nikon had a lot of room to jump in , even with a camera that had a lot of tradeoffs and was untested, undercutting all the incumbents. Now that the entry price of everything else is around $15,000 - $18,000, much of that breathing room is gone. Nikon can't charge what any of the mature players do, unless they bring comparable flexibility in lenses and other options (back upgrades and service). This board is full of stories of the medium format companies offering very good service that only a tiny company can offer - representatives of several of the manufacturers/dealers are here, and are extremely helpful (thanks Thierry and the Capture Integration team, and sorry if I missed anyone). Nikon can't match that, almost by definition - and certainly has shown no interest in it with their other cameras.
If Nikon wants to get a meaningful competitor in there, it will either have to SEVERELY undercut the existing players (body under $8000) or offer something nobody else has. Without the "body and back" design, and without service that will swap a back immediately, photographers will certainly need two of them (no more having an old AFDII or H1 on hand if the body goes and counting on a dealer's loaner if the back fails), and no using a few rolls of Velvia in the fridge as a backup either. I can't imagine any Nikon dealer stocking loaner cameras, and Nikon's repair turnaround runs several weeks (so does Canon's, and everyone else's in 35mm). The only upgrade to the latest version will be to sell the whole body and buy the next generation. All of this put together means that a Nikon entry will have to sell for substantially less (thousands, but no idea how many) than any of the existing cameras with similar image quality, even if they can match the lens lineup.
If Nikon tries simply undercutting the other guys, does this jam up the top of their line with the D3 at $5000 - where can they get a D3x in there (unless there IS no D3x and MX is their only high-pixel count play)? If they really are developing a "giant rangefinder" for $8000 or so, that's probably a better bet, because there is nothing like that on the market. They might also do well with something that was more conventional, but had 35mm-caliber autofocus (all of the existing cameras have different variants of "focus and recompose" AF).
Since I actually like shooting medium format better than 35mm (as a landscape photographer), I'm working on a switch from Canon to one of the medium format systems (which I could never have dreamed of before last week's price drops). I would love to see the MX before I do, or at least have a better sense of what it might be. Nikon's ergonomics are often amazing, and I think the giant rangefinder could be better suited to what I do than even a MF SLR, but I'd rather have a camera that handles like a Hasselblad than one that handles like a giant 35mm SLR - strictly one opinion, for one type of photography. One that handles like a Mamiya 7 plus a hint of view camera could be even better.
Could Mamiya be in a position to release that giant rangefinder? The only reason I mentioned Nikon is that there is a persistent rumor that they are working on it. Logically, Mamiya is a much better fit - having both medium format and rangefinder experience, where Nikon has neither.
-Dan