  We study lens statistics in flat, low-density universes with different
equations of state $w=p_Q/\rho_Q$ for the dark energy component. Dark energy
modifies the distance-redshift relation and the mass function of dark matter
halos leading to changes in the lensing optical depth as a function of image
separation. Those effects must, however, be distinguished from effects
associated with the structure of dark matter halos. Baryonic cooling causes
galaxy-mass halos to have different central density profiles than group- and
cluster-mass halos, which causes the distribution of normal arcsecond-scale
lenses to differ from the distribution of ``wide-separation'' ($\Delta\theta
\gtrsim 4\arcsec$) lenses. Fortunately, the various parameters related to
cosmology and halo structure have very different effects on the overall image
separation distribution: (1) the abundance of wide-separation lenses is
exremely sensitive (by orders of magnitude) to the distribution of
``concentration'' parameters for massive halos modeled with the
Navarro-Frenk-White profile; (2) the transition between normal and
wide-separation lenses depends mainly on the mass scale where baryonic cooling
ceases to be efficient; and (3) dark energy has effects at all image separation
scales. While current lens samples cannot usefully constrain all of the
parameters, ongoing and future imaging surveys should discover hundreds or
thousands of lenses and make it possible to disentangle the various effects and
constrain all of the parameters simultaneously. (abridged)
