  Using an isospin- and momentum-dependent transport model we study
near-threshold pion production in heavy-ion collisions induced by radioactive
beams at the planned Rare Isotope Accelerator (RIA). We revisit the question of
probing the high density behavior of nuclear symmetry energy $E_{sym}(\rho)$
using the $\pi^-/\pi^+$ ratio. It is found that both the total and differential
$\pi^-/\pi^+$ ratios remain sensitive to the $E_{sym}(\rho)$ when the
momentum-dependence of both the isoscalar and isovector potentials are
consistently taken into account. Moreover, the multiplicity and spectrum of
$\pi^-$ mesons are found more sensitive to the $E_{sym}(\rho)$ than those of
$\pi^+$ mesons. Finally, effects of the Coulomb potential on the pion spectra
and $\pi^-/\pi^+$ ratio are also discussed.
