Difficult to predict. Product, advertising, studio work, high-end weddings and stock photography are about all that require much over 8MP or 10MP. Not really the traditional hunting grounds of Nikon.
These are the traditional specialties of people shooting medium and large format film, and are now dominated by digital backs on medium format bodies and scanning backs on view cameras. Canon may have over ran its headlights into an area beyond where sales are easy. These people are not at all accustomed to working with a 35mm chassis, are content with their Hasselblads and Mamiyas, and may have no interest in changing.
Few photographers outside of studios would want to put up with such huge image files. If I were shooting landscapes for extremely large prints, I would probably go with a field camera and a scanning back. I could see wedding photographers who specialize at the high-end perhaps going for one, but they would already be mostly shooting with Hasselblads and the like. Very large prints are quite common when shooting weddings of the rich and famous.
The D3 is clearly aimed first at photojournalists and sports shooters, with a nod to portrait studios. I expect that a more general purpose D3X will be released early next year, spanning the range of high-end general purpose shooting with some cameras going into lower end product, advertising and the like.
Commercial shooters are finding more and more of their work going to the web, and shooting a 60MB 5616 x 3744 image and having it reduced to a 64k 640 x 480 is beyond ludicrous. It also takes a fast machine, very rich in RAM to deal with such large images with efficiency.
A lot of working shooters are asking for fewer pixels and much better quality pixels. The D3 appears to address this, and from reports of those who have shot with the early versions, this seems to be the result. The editor of Imaging Resources was at the Tokyo introduction and reported that there was no visible noise at ISO1600. This is far more impressive than resolution. I expect that editors and art directors will adore the output of the D3. I also expect that a lot of working photographers and well heeled enthusiasts will be drawn to the D3X or whatever Nikon may call it when it ships.
Canon may have created a bit of a white elephant.