Combine the sugar with 1 cup water in a small saucepan. Bring to a boil and simmer for 2 minutes, until the sugar dissolves. Pour into a 9 x 13 x 2-inch rectangular (non-metal) baking dish. Stir in the grapefruit juice, Champagne, and fleur de sel.
Carefully place the dish on a level surface in the freezer. Freeze for one hour, rake the granita with a large dinner fork, and return it to the freezer. Continue to rake the granita every 30 minutes, until it's firm and granular. This might take as long as 3 hours. Serve frozen in pretty bowls or stemmed glasses.