Hi fullhamish:
"In your world(s), where morality is subjective, might I ask just how much these high-flown ideals amount to? Are they to be discarded when environmental stresses reach a high enough pitch?"
I won't say that most individuals will discard them. Most will not. But those individuals who have a morality more in line with said environmental stresses will be the ones who pass their morality on to their children. Morality absolutely evolves over time, for the faithful as well as atheists, whether they admit it or not. I don't see many Christians making moral arguments for slavery today, for example. Those unfortunate bible passages that support slavery as a moral system are brushed aside, ignored, swept under the rug by all mainstream Christians. 100 years from now you won't see them making moral arguments against homosexual marriage, either. Over the long term, reality shapes morality, not the inverse.
On a side note, please do remember that whether or not a person believes morality is subjective, they are equally likely to act against their own morale dictates. In other words, Christians rape, murder, and abort at the same rates as atheists.