We study the Casimir force acting on a conducting piston with arbitrary cross
section. We find the exact solution for a rectangular cross section and the
first three terms in the asymptotic expansion for small height to width ratio
when the cross section is arbitrary. Though weakened by the presence of the
walls, the Casimir force turns out to be always attractive. Claims of repulsive
Casimir forces for related configurations, like the cube, are invalidated by
cutoff dependence.Comment: An updated version to coincide with the one published December 2005
in PRL. 4 pages, 2 figure