Submillimeter Observations of an Asymmetric Dust Disk around Fomalhaut

New submillimeter images of the cold dust emission around the nearby main-sequence star Fomalhaut are presented. Observations at a wavelength of 450 (mu)m, where the telescope beam size is equivalent to a resolution of 50 AU, reveal that Fomalhaut is encircled by a significantly nonaxisymmetric inclined ring. Smooth axisymmetric models of the ring images suggest the existence of a least one "clump" with an estimated flux of about 5% of the total from the disk, thus implying a clump mass of 0.075 lunar masses. At the resolution of the data, this clump could instead be a ring arc. The most plausible explanation is that this feature is produced by dust trapped in a resonance with a large planet. The observed structures around Fomalhaut and other Vega-excess stars qualitatively resemble features seen in numerical simulations with a gas giant perturber.