Can an Atheist Justify Morality?

Religious people are seen by some atheists as committing an intellectual, and perhaps moral, offense by making unjustifiable claims and leading others to believe them.  They point  out that the existence of God is unfalsifiable and assert that there is not a single good reason for believing it.

It's an arguable point, worth discussing.  Personally, it strikes me as a rather cynical and dim view of humanity that the overwhelming majority of people structure their lives around a belief that there isn't a shred of evidence for.  But that's beside the point.

What I really want to point out is that many of these same atheists seem to think that unjustifiable claims are okay when it comes to talking about morality.  Sam Harris, for example, seems to constantly brush off anyone who presses him to justify his moral positions.  For instance, in response to the idea that we need some type of religion to say that what the Nazis did was objectively wrong, Harris does 3 things:

1) He says that we have, "very serviceable intuitions about good and evil and what constitutes an ethical life, and we converge on those intuitions."

2) He points out religion's role as an especially insidious source of division amongst humanity, due to its ability to locate the source of that division in a transcendent being, as a reason why religion can't possibly be a valid guide to morality.

3) He talks about the Bible's track record on slavery as an example of further proof that religion can't possibly be  a valid guide to morality.  He concludes this point by adding that even in the implausible scenario that the abolitionists were inspired by Scriptural principles, that still would not proove that the book is divinely authorized.

I just wanted to point out that not a single one of these things constitutes even an attempt to answer the question of why, in the absence of a creator God, should we value the wellbeing of ourselves and/or others.  It seems that in response, Harris and many other atheists are content to say things like, "It's obvious," and like to point out that the overwhelming majority of us just do value those things.  They claim that justification for this belief is neither necessary nor particularly useful.  Anywho who would argue otherwise obviously has some sort of mental defect and can't really be reasoned with.

That sounds great, but I'm curious as to how this is not a double standard when someone also conceives a theist who seems to have little justification for belief in God as guilty of intellectual and moral failure.  How can one person be so demanding for justification in regards to theism, yet simultaneously excuse himself from any necessity for justification in regards to ultimate moral principles?

Another claim that Harris is fond of making is that Christian abolitionists were on the losing side of a theological argument with Christian slaveholders.  While I disagree with that, it's tempting to play Harris's game.  One could make a good case that Harris and other morally-driven atheists are really on the losing side of a philosophical argument with those atheists who have embraced the cold, hard reality of moral relativism in a godless universe.