  Using a Jacobi-Trudi-type identity, we show that the scalar product of a
general state and a Bethe eigenstate in a finite-length XXZ spin-1/2 chain is
(a restriction of) a KP tau function. This leads to a correspondence between
the eigenstates and points on Sato's Grassmannian. Each of these points is a
function of the rapidities of the corresponding eigenstate, the inhomogeneity
variables of the spin chain and the crossing parameter.
