Dark matter halos formed in CDM simulations are known to have triaxial shapes, with substantial distortions from spherical symmetry. However, conflicting claims have been made with respect to radial variations of halo shape, or trends with mass. We propose a new, more robust method to determine halo shapes based on the gravitational potential, thereby avoiding measurement uncertainties that may be the cause of these discrepancies. We find a strong preference towards prolate halo shapes, with mean minor-to-major axis-ratios close to ~ 0.5. Spherical halos are generally very rare, but there is a trend for smaller mass systems to be less elongated. We also compare dark halo shapes found in hydrodynamical simulations that include radiative cooling, star formation, and feedback processes, with those measured in equivalent collisionless simulations. Dissipation makes dark halos substantially rounder at small radii, to the extent that their shape distribution matches that of elliptical galaxies.