  We consider the holographic principle, in its lightsheet formulation, in the
semiclassical context of statistical-mechanical systems in classical Einstein
spacetimes. A local condition, in terms of entropy and energy local densities
of the material medium under consideration, is discussed, which turns out to be
necessary and sufficient for the validity of the closely-related generalized
covariant entropy bound. This condition is apparently a general consequence or
expression of flat-spacetime quantum mechanics alone, without any reference to
gravity. Using it, a lower bound \eta/s >= 1/4\pi can be derived, with the
limit attained (in certain circumstances) by systems hydrodynamically dominated
by radiation quanta.
