In thinking about the relation between ethics and theology, it helps to distinguish the metaphysical aspects of this problem from the epistemological ones. Or, as St. Thomas would say, the order of being from the order of knowing. Value, or ethics, may depend metaphysically on the existence of God, but that doesn’t necessarily mean that knowing moral truths requires knowing (or believing) that God exists.
It seems to me that, difficult moral dilemmas aside, I have no particular difficulty discerning my day-to-day moral duties to family and friends, co-workers, strangers, etc. (I may have difficulty doing my duty, but that’s a separate issue.) Likewise, I have no great difficulty in telling good apart from bad, at least in the normal run of things. By contrast, I have very little idea of how to tie these perceptions and duties together into some grand unified theory of ethics or how such a theory is, or should be, grounded in an overall metaphysical view of the world.
Since at least some moral truths are far more obvious than God’s existence, it’s odd to suggest that you can’t have ethics without God. After all, that would imply that the more certain has to be based on the less certain. (Well, maybe there are people for whom the existence of God is more self-evident than their normal moral duites, but I’m not one of them.) In fact, it’s plausible to think that if God created us and wants us to be good, he would make it possible for us to know moral truths even without believing in him.