  Recently, ATIC and PAMELA measured an anomalously large flux of leptonic
cosmic rays which may arise from dark matter self-annihilation. While the
annihilation signal predicted for a smooth halo is 10^2-10^3 times smaller than
the measured excess, the signal can be boosted by the presence of subhalos. We
investigate the feasibility of large boost factors using a new Monte Carlo
calculation technique that is constrained by previous simulation work on halo
substructure. The model accounts for the observed decrease in the amount of
substructure with decreasing halo mass and the scatter in halo structural
parameters such as the density concentration parameter. Our results suggest
that boost factors of the order of 10^2 are ruled out at more than 14 sigma. We
conclude that substructure alone, at least with commonly assumed annihilation
cross-sections, cannot explain the anomalous flux measured by ATIC and PAMELA.
