  We have calculated the rate coefficients for D(1s) + H^+ <--> D^+ + H(1s)
using recently published theoretical cross sections. We present results for
temperatures T from 1 K to 2x10^5 K and provide fits to our data for use in
plasma modeling. Our calculations are in good agreement with previously
published rate coefficients for 25 <= T <= 300 K, which covers most of the
limited range for which those results were given. Our new rate coefficients for
T >~ 100 K are significantly larger than the values most commonly used for
modeling the chemistry of the early universe and of molecular clouds. This may
have important implications for the predicted HD abundance in these
environments. Using our results, we have modeled the ionization balance in high
redshift QSO absorbers. We find that the new rate coefficients decrease the
inferred D/H ratio by <~ 0.4%. This is a factor of >~ 25 smaller than the
current >~ 10% uncertainties in QSO absorber D/H measurements.
