The only way that Nikon would start a MF line, is if there actually were buyers for it, and if it wouldn't cannibalise their existing line of products.
Of course the end profit would have a saying, but I think Nikon could afford a break-even start off, if they though it would strengthen their brand, and if the future pricing of sensors would get lower.
The tendencies in the marked is that the consumer in general speaks of big sensors, and is getting a more "educated" idea of how the sensor size plays a big role for a noise free, and high dynamic image.
And look at the "full frame" development the last 3 years, Nikon has introduced the d700 at a fairly low price, and we will without a doubt see even cheaper full frame cameras within the upcoming 2 years.
Off course this turnover and volume of big sensors pushes the production prices down, and the opportunity to produce even bigger sensors at a more reasonable price becomes possible.
I mean, if Nikon could create a new MF mount that in addition will work with different adaptors, that could fit Mamiya, Hbl, Rollei, etc, plus introduce 5 new lenses, they'd be cutting quite a big piece of the market I reckon.
This is just speculation, but who knows? Nikon and Canon could afford such a step, but the question remains, can they make it profitable in the length?