  Based on a generalization of the Lifshiftz theory we calculate Casimir forces
involving magnetodielectric and possibly anisotropic metamaterials, focusing on
the possibility of repulsive forces. It is found that Casimir repulsion
decreases with magnetic dissipation, and even a small Drude background in
metallic-based metamaterials acts to make attractive a Casimir force that would
otherwise be predicted to be repulsive. The sign of the force also depends
sensitively on the degree of optical anisotropy of the metamaterial, and on the
form of the frequency-dependency of the magnetic response.
