The groom had taken his place at the altar. The soloist was singing "Ave Maria." And in the air at the Episcopal Church of Bethesda-by-the-Sea in Palm Beach, Fla., the scent of gardenias and roses lent a heady sweetness. Then shortly after 7 p.m. Jan. 22, the doors at the back of the chapel opened, and some 450 guests—from Hillary Clinton to Katie Couric to Billy Joel—were riveted by the sight of Melania Knauss, standing tall and serene in a custom-made Christian Dior gown.