  The depletion of lithium during the pre-main sequence and main sequence
phases of stellar evolution plays a crucial role in the comparison of the
predictions of big bang nucleosynthesis with the abundances observed in halo
stars. In the past a wide range of possible depletion factors, ranging from
minimal in standard (non-rotating) stellar models to as much as an order of
magnitude in models including rotational mixing have been suggested. Using
recent progress in the study of the angular momentum evolution of low mass
stars, which now permits the construction of theoretical models that reproduce
the angular momentum evolution of low mass open cluster stars, along with the
observed distribution of initial angular momenta inferred from stellar rotation
data in young open clusters, we study lithium depletion in main sequence halo
stars. We predict a well-defined, nearly flat (in T_eff) halo lithium plateau
with modest scatter and a small population of outliers. We also examine the
relative depletions of 6Li and 7Li finding that the dispersion in the plateau
and the 6Li/7Li depletion ratio scale with the absolute 7Li depletion. Using
observational data to bound the 7Li depletion in main sequence halo stars, we
find a maximum depletion of 0.4 dex is set by the observed dispersion and the
6Li/7Li depletion ratio, and a minimum depletion of 0.2 dex is required by both
the presence of highly overdepleted halo stars and consistency with the solar
and open cluster 7Li data. The cosmological implications of these bounds on the
primordial abundance of 7Li are discussed. (Abridged)
