Bill Cosby will host a 90-minute telecast of the romantic ballet "Giselle," performed by the Dance Theater of Harlem as staged by Arthur Mitchell, Dec. 27, on NBC. Mitchell's version is set in a black society of Louisiana in the early 19th Century. Mitchell called the planned telecast a major break for Dance Theater of Harlem. "It will bring our art to the masses, and I hope it will open the doors for more cultural programming of this type." The production, first presented in London in 1984, has been performed by the Harlem company on tour in Los Angeles. The telecast on NBC will be fully sponsored by AT&T.