Nyotaimori— often translated as "female body arrangement"—is the practice of eating sushi off a woman's naked body. It is not mainstream in Japan by any stretch. Yet, as sister site Jezebel points out , the tradition survives in the West.
During the 1990s, thanks to Hollywood movies like Rising Sun and a flurry of media coverage, the concept of nyotaimori spread abroad, where it has taken hold in more than the imagination. For example, Miami restaurant Kung Fu Kitchen & Sushi is serving up a nyotaimori special; it starts at US$500 and runs until late September. At this restaurant, nude male or female models are available to be the dinner plate of your choice. While the models at Kung Fu Kitchen & Sushi are disinfected beforehand and while their privates are covered with banana leaves, the practice of nyotaimori is highly unsanitary—which is probably one why you don't encounter this on a regular basis in Japan. (Also, sushi is not traditionally served with banana leaves in Japan, let alone naked bodies.)
Some members of Japanese organized crime are fans of nyotaimori; Tokyo Vice writer Jake Adelstein points out that while nyotaimori does happen, the practice is less popular with today's yakuza who view the whole thing as a little much—even by organized crime standards.
For the general public, nyotaimori has existed in a limited sense in the Japanese sex industry—still, it hasn't been mainstream. According to a 2009 Japan Times article, one Tokyo stripper talked about performing nyotaimori at a "happening bar", which also isn't a mainstream establishment, as part of a special event. (A "happening bar" is essentially an underground swingers club with a cover charge.) The stripper said that nyotaimori was so rare in today's Japan that the bar's owners thought this would be a good stunt to lure customers.
(Today, however, a Chinese news source ran a story that there are establishments in Tokyo, Osaka, and Kyoto that still offer nyotaimori; the article, though, is short on facts and long on pictures of a Western nyotaimori event. China outlawed the practice in 2004.)
There apparently was one nyotaimori establishment in Shinjuku's red light district during the late 1990s, which mixed raw fish and sexual services—a recipe for seriously ill customers. It soon closed before health officials could shutter the establishment.
Food laws in Japan—especially laws regarding raw food—are strict. In more and more sushi restaurants in Japan, sushi chefs are starting to wear plastic gloves when handling raw fish. And serving raw food on someone's body isn't sanitary: the body warms up the raw fish and salmonella can be a problem. This is why some nyotaimori events in the West have forced the human tables to chill their bodies so the fish doesn't spoil—something that not only sounds unappetizing, but also inhuman. That's why it's not only that the hygiene issues that also make nyotaimori unappetizing, but also the whole let's-turn-a-human-into-tableware aspect. Yet, the practice continues to fascinate outside of Japan and still pops up in Western depictions of Japan—such as the Spanish film Map of the Sounds of Tokyo. Nyotaimori is far more of a thing outside Japan than in it.
In the West, nyotaimori is promoted as "traditional Japanese culture". However in Japan, there isn't much info on the practice in the National Diet Library—putting that "traditional" spin into question. Japanese tabloids have a few articles here and there on it, but once again, the fringe of the sex industry is hardly "traditional Japanese culture". That's not to say it doesn't exist in Japan—it does. But nyotaimori is well outside of the mainstream of nightlife—even in the country's red light districts.
The West, however, seems to have taken to the practice. There are restaurants, such as the aforementioned Miami's Kung Fu Kitchen & Sushi, that openly offer nyotaimori specials. So if you are eager to eat raw fish off a woman (or a man), you'll probably have an easier time finding an establishment outside Japan. Just don't think that you're experiencing "traditional Japanese culture", especially when dinning at a restaurant named after a Chinese martial art that serves sushi with banana leaves.