  Collisional excitation of the 21cm HI hyperfine transition is not strong
enough to thermalize it in warm neutral (``intercloud'') interstellar gas,
which we show by simultaneously solving the equations of ionization and
collisional equilibrium under typical conditions. Coupling of the 21cm
excitation temperature and local gas motions may be established by the Ly-alpha
radiation field, but only if strong Galactic Ly-alpha radiation permeates the
gas in question. The Ly-alpha radiation tends to impart to the gas its own
characteristic temperature, which is determined by the range of gas motions
that occur on the spatial scale of the Ly-alpha scattering. In general, the
calculation of H I spin temperatures is a more difficult and interesting
problem than might have been expected, as is any interpretation of H I spin
temperature measurements.
