In the popular religion debate, one often faces the frustrating task of talking about where our morality comes from. The difficulty of this discussion arises from a lack of common understanding of the word "moral." There are two very different, but not mutually exclusive, understandings of morals, and when they get confused, the debate goes around in circles. This explains why atheists invent slogans like "good without God" and why Christians continue to claim that without God all things are permissible.
One understanding of morality is exclusively political. (Edit: I suppose "social" might be the better word here, but I prefer "political" because it indicates that people are subject to rules, not merely interconnected. Still, one should not necessarily think merely of government, but of all social pressure to adjust one's behavior.) Under this view, individuals are subject to certain rules because they are accountable to people around them. Put an individual in total isolation from everyone else, and there is no reason to talk about morality. On the other hand, put an individual in society, and talking about morals still makes sense even without divinity: morals are the rules we follow in order that we, our at least the community as a whole, may survive.
A compatible yet fundamentally different understanding of morality is one that holds each individual to absolute moral standards, regardless of his relative isolation to the community. Thus Jesus insisted that even a man who looks at a woman with lust has already committed adultery: the outward act is not the only thing that is blameworthy. In this view the life of each individual has moral value on its own, and morality is not strictly about existing in community, but also about developing one's individual life in the right way, becoming what one is "meant" to be. To have such an idea of individual development presupposes the existence of God, since otherwise individual development need not have any particular objective goals. Someone who holds this view of morality will often (thought not always) function in society as if he held the first view.
Those who hold the second view naturally argue that without God, morality is meaningless. Those who hold the first view counter that living in a community requires the adoption of certain standards, and they are right. One might object that such an argument presupposes the objective value of the survival of the community, which in turn implies the existence of God, the source of objective morals. But this presupposition is not needed for the first view to work. The survival of the community doesn't really need to be justified. If it survives by following certain moral customs, those moral customs will remain. If it doesn't survive, then the morals it has imposed on itself will become irrelevant. It seems to difficult to genuinely dispute this, other than by insisting that the definition of "morals" in operation is simply wrong. I suppose the practical question of how to convince members of the community to follow a certain rule is still relevant to some extent, but on the other hand, morals in the purely political sense need not require any argument--they will naturally be more or less agreed upon by people who already know how to live peaceably with one another. If those morals weren't already shared, then of course, the community wouldn't exist!
(In other words, we need not suppose the community exists prior to the morals they hold; rather, the community is largely defined by the morals they share. Or, to be more precise, the community is defined by the fact that they coexist through cooperation, and this cooperation requires that they adhere to some rules; whatever these rules can be called "morals," in the purely political sense. Such morals need not be thought of as morals; they need not be explicitly acknowledged or even known by anyone in the community. They are simply descriptions of those behaviors which enable that cooperation on which their survival depends.)
Those holding the second view, on the other hand, see no reason to be impressed with anyone's "goodness," if being good is meant in a purely political sense, i.e. if being good simply means acting in accord with standards which are wholly dependent on which society one happens to be born into. It does not make any difference whether there are some morals which happen to "work" better than others, in the sense that they will enable the survival of the community more easily than others. Morality in the second sense has to do even with what one does in the privacy of his own home, even when entirely alone--even behaviors which affect no one but himself. Morality in this sense is only attained when an individual reaches for a standard independent of all communities and all other individuals. In this sense, there is no way one can be "good without God," because there is no accountability--in fact, there is simply no standard to reach for.
As far as I can see, these two views of morality are philosophically incompatible, and as long as we hold to two definitions of the same word, our debate can never be fruitful.