  We study the geometry dependence of the Casimir energy for deformed metal
plates by a path integral quantization of the electromagnetic field. For the
first time, we give a complete analytical result for the deformation induced
change in Casimir energy \delta\cal E in an experimentally testable, nontrivial
geometry, consisting of a flat and a corrugated plate. Our results show an
interesting crossover for \delta\cal E as a function of the ratio of the mean
plate distance H, to the corrugation length \lambda: For \lambda \ll H we find
a {\em slower} decay \sim H^{-4}, compared to the H^{-5} behavior predicted by
the commonly used pairwise summation of van der Waals forces, which is valid
only for \lambda \gg H.
