Leica will soon have a new 24 megapixel full-frame digital rangefinder that only captures black-and-white images: the M Monochrom Typ 246 (which replaces the old M Monochrom--no confusion, right?). I think the idea is brilliant. I want a digital camera that only captures black-and-white images (of course I will not pay Leica's way-too-high price tag to get one).
Curiously, the base ISO for the M Monochrom Typ 246 is 320, which seems quite high. I understand that black-and-white photographs often look better a tad grainy, but ISO 320 is a bit ridiculous for a base ISO. Perhaps it is much cleaner than I'm picturing in my mind.
What makes a black-and-white-only camera a good idea? Well, because of how the Bayer sensors works. You see, about half of the pixels are used for color information and not luminance. Much of the data becomes (somewhat) useless for monochrome. What Leica has done with the M Monochrom cameras is allowed a much more effective use of the light data. In the end you get a (slightly) superior product pound-for-pound (or pixel-for-pixel). Oh, and you can use colored filters the same way as you would with black-and-white film.
The other option is to use a Foveon sensor. These have serious high-ISO limitations and other quirkinesses, but otherwise will achieve the same thing as the M Monochrom cameras--at a tiny fraction of the price, too. Plus, it is still very much capable of capturing in color.
I wish that Nikon or one of the other big camera companies (perhaps Fuji?) would make a similar product (but, obviously, much more affordable than a Leica). A black-and-white-only DSLR would be pretty cool. Unfortunately, I think it is such a niche market that I doubt it will ever happen.