  Thermal fluctuations in non-equilibrium steady states generically lead to
power law decay of correlations for conserved quantities. Embedded bodies which
constrain fluctuations in turn experience fluctuation induced forces. We
compute these forces for the simple case of parallel slabs in a driven
diffusive system. The force falls off with slab separation $d$ as $k_B T/d$ (at
temperature $T$, and in all spatial dimensions), but can be attractive or
repulsive. Unlike the equilibrium Casimir force, the force amplitude is
non-universal and explicitly depends on dynamics. The techniques introduced can
be generalized to study pressure and fluctuation induced forces in a broad
class of non-equilibrium systems.
