  A general theory of the Casimir-Polder interaction of single atoms with
dispersing and absorbing magnetodielectric bodies is presented, which is based
on QED in linear, causal media. Both ground-state and excited atoms are
considered. Whereas the Casimir-Polder force acting on a ground-state atom can
conveniently be derived from a perturbative calculation of the atom-field
coupling energy, an atom in an excited state is subject to transient force
components that can only be fully understood by a dynamical treatment based on
the body-assisted vacuum Lorentz force. The results show that the
Casimir-Polder force can be influenced by the body-induced broadening and
shifting of atomic transitions - an effect that is not accounted for within
lowest-order perturbation theory. The theory is used to study the
Casimir-Polder force of a ground-state atom placed within a magnetodielectric
multilayer system, with special emphasis on thick and thin plates as well as a
planar cavity consisting of two thick plates. It is shown how the competing
attractive and repulsive force components related to the electric and magnetic
properties of the medium, respectively, can - for sufficiently strong magnetic
properties - lead to the formation of potential walls and wells.
