  We present a preliminary calculation of B_K using domain-wall valence quarks
and 2+1 flavors of improved staggered sea quarks. Both the size of the residual
quark mass, which measures the amount of chiral symmetry breaking, and of the
mixed meson splitting Delta_mix, a measure of taste-symmetry breaking, show
that discretization effects are under control in our mixed action lattice
simulations. We show preliminary data for pseudoscalar meson masses, decay
constants and B_K. We discuss general issues associated with the chiral
extrapolation of lattice data, and, as an example, present a preliminary chiral
and continuum extrapolation of f_pi. The quality of our data shows that the
good chiral properties of domain-wall quarks, in combination with the light sea
quark masses and multiple lattice spacings available with the MILC staggered
configurations, will allow for a precise determination of B_K.
