  We study two systems of BPS solitons in which spin-spin interactions are
important in establishing the force balances which allow static, multi-soliton
solutions to exist. Solitons in the Israel-Wilson-Perjes (IWP) spacetimes each
carry arbitrary, classical angular momenta. Solitons in the Aichelburg-Embacher
"superpartner" spacetimes carry quantum mechanical spin, which originates in
the zero-modes of the gravitino field of N=2 supergravity in an extreme
Reissner-Nordstrom background. In each case we find a cancellation between
gravitational spin-spin and magnetic dipole-dipole forces, in addition to the
usual one between Newtonian gravitational attraction and Coulombic
electrostatic repulsion. In both cases, we analyze the forces between two
solitons by treating one of the solitons as a probe or test particle, with the
appropriate properties, moving in the background of the other. In the IWP case,
the equation of motion for a spinning test particle, originally due to
Papapetrou, includes a coupling between the background curvature and the spin
of the test particle. In the superpartner case, the relevant equation of motion
follows from a kappa-symmetric superparticle action.
