Don't know about some of this stuff, and I guess that is why it is rumor ;-) The rumor ad leaks that Lars pointed to do not suggest a square sensor (BIG ads on that site). One could just as easily conclude that "BIG" refers to a larger LCD, though I hope that is not what Nikon is touting, even if it would be very nice on some models.
Again, not looking to dampen any enthusiasm or excitement, as I think it would be very interesting to see how companies like Nikon or Canon may try to approach moving up market into some realm of MF. I think they could do it. They may have things in the works. I also think that it will not be a "cake walk" for either, despite their present market shares and such in the 35mm segment. There is a lot of clamoring from discussions on our sorts of forums, but one has to wonder just how big the potential market would really be, and what folks would really be willing to pay, and how much of a major transformation in manufacturing would be required for things like a line of lenses. In some cases, with the huge growth of more recent DSLRs, the manufacturers may be scrambling to keep up with lens demand on some of the newer things. I just really do not know capacities and capabilities of them to know how easy or hard it would be to field a completely new and much different line of things while they may be all out keeping up with the other new stuff they are pumping out.
Assuming Nikon (or Canon) has some sort of larger than 35mm sensor in the wings, it most likely will not be terribly cheap for either, until there is some projection of enough volume to sustain things. If the goal is not so much to try to carve market away from the present MF segment, but to create new market growth or a new market segment that folks are calling the "tweener" segment, that could be a different and possibly attractive segment. Drawing sales from both existing MF folks that want a faster, smaller, more feature updated camera to deliver more than present DSLRs do today, as well as drawing folks from the higher end DSLR market that are looking for more resolution, DR, and things more associated with MF, but not wanting to give up the features of present DSLRs. Trying to build a new MF back and system approach like already exists would not seem to cut it, and probably why Leica has taken the approach they have. Pricing is still going to become a sensitive point. Needs to be attractive and match the perceived value. Support and service must also be put into place, and while that may seem easier, it may not be easy to match the demands of this group of customer expectations, especially on much more expensive gear that is going to operate a bit differently than the present DSLRs that are being churned out.
That can go from interesting to becoming a very tall order for anybody. Leica has sort of fired an attractive shot across the bow. Nikon and Canon could follow, but there are going to be some interesting new bars set with respect to quality, optics, utility, design, and things like that. Both seem able to live up to the challenges, (still hoping for some significant improvements in optics however), but there has to be a big enough projected market into which to try to launch this sort of thing, I think.
LJ