  We show that, unlike other pure b -> d penguin processes, the decay Bs(t) ->
phi K_s is dominated by a single amplitude, that of the internal t-quark. The
contributions of the u- and c-quark operators each vanish due to a cancellation
between the (V-A) \otimes (V-A) and (V-A) \otimes (V+A) matrix elements. Thus,
the indirect CP asymmetry in this decay probes sin 2\beta. Although this
cancellation is complete only for certain values of the s- and b-quark masses,
the theoretical uncertainty on sin 2\beta is still less than 10% over most
(~80%) of the parameter space. By measuring the direct CP asymmetry, one can
get a better idea of the probable error on sin 2\beta.
