Do you mean placenta over the scar or over the cervix (placenta previa)? I am not aware of any research about the likelihood of problems with the placenta being over the scar but not the cervix. When the placenta overlays the cervix, a planned cesarean is often necessary, because as the cervix dilates, the placenta will hemorrhage. Sometimes, though, depending on how much of the cervix is covered, it is possible to make it through to a vaginal birth. You wouldn't want to try this outside of a hospital that could perform an urgent c/sec at any time.

-- Henci

P.S. Important note: you cannot tell until toward the end of pregnancy if placenta previa is going to pose a problem. Many a previa disappears as the pregnancy advances. As the uterus grows larger, the placenta is often carried upward sufficiently far that it moves away from the cervix.