  We reconsider the recently proposed nonlinear QED effect of quantum
reflection of photons off an inhomogeneous strong-field region. We present new
results for strong fields varying both in space and time. While such
configurations can give rise to new effects such as frequency mixing, estimated
reflection rates based on previous one-dimensional studies are corroborated. On
a conceptual level, we critically re-examine the validity regime of the
conventional locally-constant-field approximation and identify kinematic
configurations which can be treated reliably. Our results further underline the
discovery potential of quantum reflection as a new signature of the
nonlinearity of the quantum vacuum.
