  We briefly discuss the predictions of the heavy quark effective theory for
the semileptonic decays of a heavy pseudoscalar to a light one, or to a light
vector meson. We point out that measurement of combinations of differential
helicity decay rates at Cleo-c and the $B$ factories can provide a model
independent means of extracting the ratio
  $|V_{ub}|/|V_{cd}|$. We briefly discuss the corrections to this prediction.
