In the movie "Things to do in Denver when you're Dead", buckwheats refers to a particularly nasty type of assassination, characterized by a gun penetrating the anus before it is discharged.
This is never shown in the movie (thank god), and in the DVD version, the makers of the movie explain that they just made up a number of the supposed slang terms.
Buckwheat is used in soba noodles in Japan, making foods like yakisoba. The husks of buckwheat is then used to make pillows by the name of sobakawa, I think.
It is used in American food in the same way that whole-grain wheat is used, in pancakes, waffles, and muffins. There was once a breakfast cereal call Buc-Wheats I think...
Some people have an adverse reaction to buckwheat. It causes a rash in some people.