  Primordial inhomogeneous magnetic fields of the right strength can leave a
signature on the CMB temperature anisotropy and polarization. Potentially
observable contributions to polarization B-modes are generated by vorticity and
gravitational waves sourced by the magnetic anisotropic stress. We compute the
corresponding CMB transfer functions in detail including the effect of
neutrinos. The shear rapidly causes the neutrino anisotropic stress to cancel
the stress from the magnetic field, suppressing the production of gravitational
waves and vorticity on super-horizon scales after neutrino decoupling. A
significant large scale signal from tensor modes can only be produced before
neutrino decoupling, and the actual amplitude is somewhat uncertain. Plausible
values suggest primordial nearly scale invariant fields of ~ 10^(-10)G today
may be observable from their large scale tensor anisotropy. They can be
distinguished from primordial gravitational waves by their non-Gaussianity.
Vector mode vorticity sources B-mode power on much smaller scales with a power
spectrum somewhat similar to that expected from weak lensing, suggesting
amplitudes ~ 10^(-9)G may be observable on small scales for a spectral index of
n ~ -2.9. In the appendix we review the covariant equations for computing the
vector and tensor CMB power spectra that we implement numerically.
