  We calculate leading-order dilepton yields from a quark-gluon plasma which
has a time-dependent anisotropy in momentum space. Such anisotropies can arise
during the earliest stages of quark-gluon plasma evolution due to the rapid
longitudinal expansion of the created matter. A phenomenological model for the
proper time dependence of the parton hard momentum scale, p_hard, and the
plasma anisotropy parameter, xi, is proposed. The model describes the
transition of the plasma from a 0+1 dimensional collisionally-broadened
expansion at early times to a 0+1 dimensional ideal hydrodynamic expansion at
late times. We find that high-energy dilepton production is enhanced by
pre-equilibrium emission up to 50% at LHC energies, if one assumes an
isotropization/thermalization time of 2 fm/c. Given sufficiently precise
experimental data this enhancement could be used to determine the plasma
isotropization time experimentally.
