  We review the reasons that make superconductivity unlikely to arise in a
ferromagnet. Then, in light of the report by Tallon and collaborators that
RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below
the Curie temperature of 132 K, we consider whether the objections really apply
to this compound. Our considerations are supported by local spin density
calculations for this compound, which indeed indicate a ferromagnetic RuO2
layer. The Ru moment resides in t_2g orbitals but is characteristic of
itinerant magnetism (and is sensitive to choice of exchange-correlation
potential and to the atomic positions). Based on the small exchange splitting
that is induced in the Cu-O layers, the system seems capable of supporting
singlet superconductivity an FFLO-type order parameter and possibly a pi-phase
alternation between layers. If instead the pairing is triplet in the RuO2
layers, it can be distinguished by a spin-polarized supercurrent. Either type
of superconductivity seems to imply a spontaneous vortex phase if the
magnetization is rotated out of the plane.
