  We derive an expression for the collective Casimir-Polder interaction of a
trapped gas of condensed bosons with a plane surface through the coupling of
the condensate atoms with the electromagnetic field. A systematic perturbation
theory is developed based on a diagrammatic expansion of the electromagnetic
self-energy. In the leading order, the result for the interaction-energy is
proportional to the number of atoms in the condensate mode. At this order,
atom-atom interactions and recoil effects lead to corrections compared to the
single-atom theory, through shifts of the atomic transition energies. We also
discuss the impact of the spatial delocalization of the condensate mode.
