  Galaxy-scale strong gravitational lenses with measured stellar velocity
dispersions allow a test of the weak-field metric on kiloparsec scales and a
geometric measurement of the cosmological distance-redshift relation, provided
that the mass-dynamical structure of the lensing galaxies can be independently
constrained to a sufficient degree. We combine data on 53 galaxy-scale strong
lenses from the Sloan Lens ACS Survey with a well-motivated fiducial set of
lens-galaxy parameters to find (1) a constraint on the post-Newtonian parameter
gamma = 1.01 +/- 0.05 and (2) a determination of Omega_Lambda = 0.75 +/- 0.17
under the assumption of a flat universe. These constraints assume that the
underlying observations and priors are free of systematic error. We evaluate
the sensitivity of these results to systematic uncertainties in (1) total
mass-profile shape, (2) velocity anisotropy, (3) light-profile shape, and (4)
stellar velocity dispersion. Based on these sensitivities, we conclude that
while such strong-lens samples can in principle provide an important tool for
testing general relativity and cosmology, they are unlikely to yield precision
measurements of gamma and Omega_Lambda unless the properties of the lensing
galaxies are independently constrained with substantially greater accuracy than
at present.
