[via a friend]
Short version: Healing is OK, as it is not "playing God"; improving species through crossbreeding (or genetic manipulation) is not OK, as it is "playing God." Abortion is OK, if it is to save the life of the mother (there's plenty of debate about what that means, but we'll move on) but not for any other reason, including saving the life of someone other than the mother. There's no reason why stem cells from adults or umbilical cords could not be used in healing; and since embryonic stem cells are not aborted, there's not much reason to think they are a violation of Rabbinic interpretations of Jewish versions of God's Law. In fact, there is some reason to think that the ability to screen embryos for genetic defects is a good thing, though I can't for the life of me figure out how that isn't "playing God."
I'm not a Rabbi, though.