The complexation and phase behavior of oppositely charged polyelectrolyte/macroion mixtures is investigated using a Poisson-Boltzmann cell model, extended to include the polyelectrolyte component at the level of a mean field treatment. We find that most of the observed features of associative phase separation in oppositely charged polymer/surfactant mixtures are recovered by this model. In addition, the model makes predictions for trends in the critical aggregation concentration in these systems which are in reasonable agreement with experiment.