No, a 50mm lens is still a 50mm lens, not a 75mm lens; it is only that the image gets cropped in the D100.
Depth of field (and thus hyperfocal distance) is a bit more complicated than even that though since it also depends on something called "cicle of confusion." Essentially, the plane of focus is always just a plane, but things in somewhat in front of and behind that plane can still "look like" they are in focus. If you photograph a true point source of light, in focus it will appear as a point. Slightly out of focus, it will begin to look like a disc and not so much like a point. If it is almost in focus, it may still look like a point, or it may not, depending on how close you look. Since the sensor in a digital camera is smaller than the 35mm frame size (in most cases at least), you have to enlarge the resulting image more to get results of the same final print size. Since the raw image from 35mm has to be magnified less to view the same size, 35mm images can tolerate being more out of focus and still look like they are in focus, when compared to an image from a smaller format like most DSLR's. As such, the acceptable "circle of confusion" (the size of a disc below which the point of light still looks like a point) on film/CCD is smaller for a digital camera than film.
So, what this means is that even though a 50mm lens is still a 50mm lens, it actually will give you *less* depth of field than a 50mm lens on a film camera. Of course, once you crop that image to look like you took it with a 75mm lens, you now have *more* *apparent* depth of field when comparing with a similarly cropped image from 35mm taken with a true 75mm lens. Gosh.
Bottom line, your depth of field with a 50mm lens on digital will be somewhere between that of the same lens on 35mm and that of a 75mm lens on 35mm. Given that hyperfocal distance is the point at which the far bound of your depth of field just touches infinity, it too will depend on the circle of confusion (CoC) for digital rather than 35mm.
For lots more info and charts based on different digital cameras (not all sensors are exactly the same size) check out the truly wonderful site at http://dfleming.ameranet.com - by far the best on the net for depth of field and hyperfocal calculations for film or digital.