I've got one or two posts about ethics and atheism, either in the body of the post or in the comments. You can look here. And here. (The post at that second link rambles a bit, not real happy with it.)
I basically hold that if you are an atheist, a real true atheist, then you cannot hold that there is such a thing as an objective moral order. Now, laws are a moral code, a system of ethics on the scale of a nation. Laws and other ethical systems are based on an objective moral order, a sense of "justice" that is above the law and against which we know if a law is a good law or a bad law. If there is no God, however, there is no such thing as an objective reality called "justice," but only what individuals decide for themselves.
I also hold that if an atheist asserts there is such a thing as an objective moral order, then his thinking is polluted by religiosity, having grown up in a society whose laws and sense of justice is formed by religious philosophy. A real atheist has to purify his mind and get down to brass tacks: The only thing he is, is a material body, and the only ethical system that matters is the one he chooses for himself. Things might be illegal, but immoral or unethical -- that only he can decide. Therefore, he becomes the supreme arbiter and judge of what is ethical, and law gets its authority only from the power to coerce obedience, not from any kind of objective justice to which it conforms.
But there is another way to go for such ethical atheists. Admit the fact that you believe in something higher than yourself that is not merely the source of "justice" but is in fact Justice Itself, Morality Itself, Reason Itself, Good Itself, Cooperation Itself. That is, deep down, if you believe there is an objective moral order, you really do believe in God.
Check out this final post in the atheist portal a Patheos, a renowned young atheist who went to Yale named Leah Libresco. She's way smarter and better educated than I am on the atheist-ethics thing. And she walked that walk from atheism. To Catholicism. From abject falsehood...to the objective truth.
What she realized is that the middle ground is bogus. Atheism cannot work with ethics. She thought it through. And decided that Morality (another name for God, if you think about it) exists.
H/T to the Anchoress.