  The 21cm background from the epoch of reionization is a promising
cosmological probe: line-of-sight velocity fluctuations distort redshift, so
brightness fluctuations in Fourier space depend upon angle, which linear theory
shows can separate cosmological from astrophysical information. Nonlinear
fluctuations in ionization, density and velocity change this, however. The
validity and accuracy of the separation scheme are tested here for the first
time, by detailed reionization simulations. The scheme works reasonably well
early in reionization (< 40% ionized), but not late (> 80% ionized).
