  We discuss the obstacles for defining a set of observable quantities
analogous to an S-matrix which are needed to formulate string theory in an
accelerating universe. We show that the quintessence models with the equations
of state $-1 < w <-1/3$ have future horizons and may be no better suited to an
S-matrix or S-vector description. We also show that in a class of theories with
a stable supersymmetric vacuum, a system cannot relax into a zero-energy
supersymmetric vacuum while accelerating if the evolution is dominated by a
single scalar field with a stable potential. Thus describing an eternally
accelerating universe may be a challenge for string theory as presently
defined.
