I believe in the states TV ads pronounce Nikon with a long I sound: "Nigh-con." More authoritatively, however, Simon and Garfunkel's track "Kodachrome" includes the line: "I've got at Nigh-Cone camera, I love to take photographs.." and me and every other baby boomer in the States has heard that line and pronunciation probably about 500 times over the course of our lives.
