  Recent first principles calculations of the phonon dispersion curves in MgB_2
have suggested the presence of anomalies in some of the curves, particularly in
the longitudinal acoustical (LA) branch in the Gamma to A direction. Similar
behavior has been observed in numerous other superconductors with T_c's higher
than those of standard electron-phonon BCS superconductors. Phenomenological
calculations of the Gamma->A LA dispersion based on both an acoustical plasmon
and a ``resonant polarization'' mechanism are given here to emphasize the
importance of these similarities.
