  We consider the shear flow of well-aligned one-component smectic phases, such
as thermotropic smectics and lamellar diblock copolymers, below the critical
region. We show that, as a result of thermal fluctuations of the layers,
parallel ($c$) alignment is generically unstable and perpendicular ($a$)
alignment is stable against long-wavelength undulations. We also find,
surprisingly, that both $a$ and $c$ are stable for a narrow window of values
for the anisotropic viscosity.
