  A recent theoretical calculation shows that the Casimir force between two
parallel plates can be repulsive for plates with nontrivial magnetic properties
(O. Kenneth et al., Phys. Rev. Lett. 89, 033001 (2002)). According to the
authors, the effect may be observed with known materials, such as ferrites and
garnets, and it might be possible to engineer micro- or nanoelectromechanical
systems (MEMS or NEMS) that could take advantage of a short range repulsive
force. Here we show that on the contrary the Casimir force between two parallel
plates in vacuum at micron and submicron distance is always attractive.
