  We analyze the Casimir forces for an ideal Bose gas enclosed between two
infinite parallel walls separated by the distance D. The walls are
characterized by the Dirichlet boundary conditions. We show that if the
thermodynamic state with Bose-Einstein condensate present is correctly
approached along the path pertinent to the Dirichlet b.c. then the leading term
describing the large-distance decay of thermal Casimir force between the walls
is 1/D^2 with a non-universal amplitude. The next order correction is lnD/D^3.
These observations remain in contrast with the decay law for both the periodic
and Neumann boundary conditions for which the leading term is 1/D^3 with a
universal amplitude. We associate this discrepancy with the non-zero
D-dependent positive value of the one-particle ground state energy in the case
of Dirichlet boundary conditions.
