  We study the coherent effect of the Casimir-Polder interaction on the
oscillations of two-photon driven atoms. We find that, for oscillations between
two degenerate states in lambda-configuration, shifts on the Rabi frequency may
be induced by non-additive level shifts. For oscillations between two Rydberg
states in ladder-configuration, shifts on the Rabi frequency may be induced by
the effective renormalization of the laser interaction.
