Sam Harris, author of The End of Faith and Letter to a Christian Nation, gave a recent TED talk in which he argues, against what seems to be a common consensus, that morality and moral values can be put on a firm "scientific" - meaning fact-based - ground.
Many bloggers have responded, including one of my favorite physics bloggers, Sean Carroll. Sean points out the saw, going all the way back to Hume, that one can't derive moral truths from factual statements about the world. This is often shortened to, "You can't get 'ought' from 'is'."
Harris responded to Carroll and other critics in a post at Project Reason. He quite reasonably points out that he only had 18 minutes to present his case (per TED requirements), and lays out his view in more detail. (He also points to a forthcoming book in which he will make the case in greater length.) Harris says Hume is by no means the last word on "is" and "ought", and not all philosophers accept Hume's conclusion. Luke Muelhauser makes a similar point on Common Sense Atheism.
Carroll answered Harris again here. Russell Blackford came down on Carroll's side, too, citing Peter Singer.
I have no idea who, if anyone, is right in all this. I know I would like to have an objective definition of morality, both because I would like a rational way of ordering - and defending - my own actions and because I would like a rationally defensible way of requiring others to do the same. If it's all just a matter of personal preference, then no progress can ever be made in the moral arena. But I'm skeptical of Harris's approach precisely because of that desire: my desire for an answer might mislead me into thinking that there is an answer.
At any rate it is a fascinating, and, I think, valuable debate.