  An arbitrarily small concentration of impurities can affect the spin Hall
conductivity in a two-dimensional semiconductor system. We develop a
Boltzmann-like equation that can be used for impurity scattering with arbitrary
angular dependence, and for arbitrary angular dependence of the spin-orbit
field b(k) around the Fermi surface. For a model applicable to a 2D hole system
in GaAs, if the impurity scattering is not isotropic, we find that the spin
Hall conductivity depends on the derivative of b with respect to the energy and
on deviations from a parabolic band structure, as well as on the angular
dependence of the scattering. In principle, the resulting spin Hall
conductivity can be larger or smaller than the ``intrinsic value'', and can
have opposite sign. In the limit of small angle scattering, in a model
appropriate for small hole concentrations, where the band is parabolic and b ~
k^3, the spin Hall conductivity has opposite sign from the intrinsic value, and
has larger magnitude. Our analysis assumes that the spin-orbit splitting $b$
and the transport scattering rate tau^{-1} are both small compared to the Fermi
energy, but the method is valid for for arbitrary value of b*tau.
