Ah, finally, an interesting topic to discuss. Here are my two pfennigs:
I've spent a lot of time and effort on various internet discussion Groups on this particular subject, and much offline debate and discussion as well. Much hinges on the intended meaning of the term "objective morality".
On the one paw, it could be meant descriptively or passively. For example, one could say that at precisely 2.59 PM SBT 13th December 2004, all humans on planet Earth do believe that murder is wrong, therefore it is objectively true that murder is wrong. Of course, put this way, something that is objectively true at 2.59 PM SBT 13th December 2004 is not necessarily true one minute later, and it takes only one human to change it from objectively true to objectively false. Now, I know that most people don't have this meaning in mind when they talk about "objective morality", but it is still good to have this in the open so that people don't end up talking past one another.
So, on the other paw, what people usually mean by "objective morality" is a prescriptive, active form of the term. An extreme form of this might be to say that murder is wrong, always was wrong and always will be wrong, for any and all beings that are capable of intelligent, conscious, intentional thought, whether existing now, in the past or in the future, regardless of the actual thoughts, actions and beliefs of such beings.
I guess this gives away my own position of extreme skepticism about "objective morality". Put in the prescriptive, active form outlined above, one can examine some of the properties that this "objective morality" must necessarily have in other to fit the criteria described:
1) It must be timeless and eternal.
2) It must exist externally to thinking beings and independently of them.
3) In order for it to matter in any way, it must also be somehow accessible to such thinking beings.
Of course, any claim that something exists and posseses these properties sits extremely badly with the naturalistic frame of mind. (Of course, this could be interpreted as an indictment of the validity of the naturalistic view rather than an indictment of the dubiousness of the existence of such a thing, but that is another matter.)
Hmm, I'm running out of time here. But here's a link that agrees with my own thoughts on the subject quite well:
On the Nature of Morality
And let me leave with another thing to think about
: it appears that much of mathematics and formal logic also possess the properties of timelessness etc. So if I claim that the weird properties possessed by an "objective morality" makes it suspect within a naturalistic frame work, would the same argument apply to mathematics and logic?