In addition to the Kodak DCS100 digital back, here is another way you can get "instant results" from an old Nikon film camera: the Speed Magny 100 Back can turn your Nikon F into a instant Polaroid machine. KEHblog has a detailed description and they even have two pieces in stock (price: $225-$235):
"The Speed Magny back attaches like, and in place of, the regular back. The accessory comes down from under the camera and takes instant pack films like Polaroid 107 and 108. The device takes the light and image coming through the lens and basically bounces it around a series of lenses and mirrors inside the unit and then magnifies and projects it, creating a full-frame pull-apart image (3.25 x 4.25") instead of a "small format" 35mm image on the film."
Nikon Speed Magny related links: