  We consider the physics of free precession of a rotating neutron star with an
oblique magnetic field. We show that if the magnetic stresses are large enough,
then there is no possibility of steady rotation, and precession is inevitable.
Even if the magnetic stresses are not strong enough to prevent steady rotation,
we show that the minimum energy state is one in which the star precesses. Since
the moment of inertia tensor is inherently triaxial in a magnetic star, the
precession is periodic but not sinusoidal in time, in agreement with
observations of PSR 1828-11. However, the problem we consider is {\it not} just
precession of a triaxial body. If magnetic stresses dominate, the amplitude of
the precession is not set just by the angle between the rotational angular
velocity and any principal axis, which allows it to be small without
suppressing oscillations of timing residuals at harmonics of the precession
frequency. We argue that magnetic distortions can lead to oscillations of
timing residuals of the amplitude, period, and relative strength of harmonics
observed in PSR 1828-11 if magnetic stresses in its core are about 200 times
larger than the classical Maxwell value for its dipole field, and the stellar
distortion induced by these enhanced magnetic stresses is about 100-1000 times
larger than the deformation of the neutron star's crust. Magnetic stresses this
large can arise if the core is a Type II superconductor, or from toroidal
fields $\sim 10^{14}$ G if the core is a normal conductor. The observations of
PSR 1828-11 appear to require that the neutron star is slightly prolate.
