Morality without supernatural origins

Is morality a gift from higher powers?  If those higher powers do not exist, is there any reason to be moral?  A movement to study natural morality (or humanistic morality or secular morality, etc.) is developing, and one group recently held a conference on this new science (some conference materials are here). 
As conference attendee and noted secularist Sam Harris  noted:

The failure of science to address questions of meaning, morality, and values has become the primary justification for religious faith. Even among religious fundamentalists, the defense one most often hears for belief in God is not that there is compelling evidence that God exists, but that faith in Him provides the only guidance for living a good life.

From a humanist perspective, morality is explainable without resorting to the supernatural.  There are rational reasons to behave, such as the desire to maximize the social efficiency and not be harmed ourselves.  And there are the human needs that we may call irrational:  the desire to be liked and respected, the compassion we feel for others, and the need to feel good about ourselves.  And it is up to human beings to decide what is moral and to enforce that code.  Rather than Scripture dictating morality, people have interpreted Scripture to reflect the morality of the time (e.g., slavery and stoning).  And so this naturalistic study of morality has profound relevance to our lives.