  By combining experimentally measured partial branching fractions for the
semileptonic decays D to K l nu and D to pi l nu with lattice calculations of
the form factors f_+^{D to K}(q^2) and f_+^{D to pi}(q^2), one can extract the
CKM matrix elements |V_{cs}| and |V_{cd}|. We are calculating the form factors
by using Fermilab charm and asqtad staggered light and strange quarks on 2+1
flavor asqtad staggered ensembles generated by the MILC Collaboration. We vary
the light valence quark masses from 0.4m_s to 0.05m_s (m_s is the strange
sea-quark mass), and the lattice spacings, from about 0.12 fm to about 0.045
fm. We extrapolate to the physical light-quark mass and the continuum limit
using heavy-light meson staggered chiral perturbation theory about the SU(2)
and SU(3) limits, compare the resulting (preliminary) form factor shapes with
experiment, and discuss our errors.
