Ethical relativism, in the most basic and practical sense, might be said to backfire if it can be seen as a primary challenge to the establishment of a meaningful ethical system. As pointed out in post #2, cultural relativism is a concept meant to protect one set of values from being derided or diminished by another. One culture's aesthetic should be seen as being equal to the aesthetic of another, etc.
However, if this concept is applied without restraint to the field of ethics, the basic point of an ethical system becomes undermined. If an act can be said to be ethical in one circumstance and to be criminal in another, we may run into problems distinguishing right from wrong.
Of course, this is a problem in the field of ethics without any relativism being introduced...but if the argument for relativism precedes the argument for a real sense of the possibility of knowing right from wrong, we will be stopped short of constructing a practical system of rules for behavior.
I don't disagree with anything the first post said, but I don't see that as "backfiring." To me, something backfires if it comes back to hurt the people who use it.
So, this could backfire if a person advocating ethical relativism is hurt by someone else acting on their cultural values. So this would be like what would happen if you were Catholic and you supported ethical relativism and then had someone refuse to hire you because they are culturally opposed to Catholicism. This is a case of backfire because you have no moral way to argue that their action is wrong. If all values are equally valid, values that cause others to hurt you cannot be wrong.
There are a couple of points in arguing the "backfire" to ethical relativism. The first is that it is logically inconsistent. If ethical relativism is meant to create everything as relative, this means that no absolute statements can be made. Everything is relative to one's own condition. However, this is an absolute statement. In making the argument that everything is relative and no absolute statements can be made, then one is actually making an absolute statement in saying that "there can be no absolute statements." In this light, the position is logically inconsistent. Another element behind it that represents problems from an ethical standpoint is its premise. Relativism was designed to protect cultural practices and beliefs from being judged. However, there are some cultural practices that should be criticized. While it might not be our culture, individuals should not be prevented from speaking out when atrocity is present. For example, the genocide in Darfur or the Holocaust would represent events where the ethical relativist would have to admit is "relative" to one's own context. I think that those who seek to manipulate power and control for their own benefit would love to see a relativist approach adopted as it "strengthens the aggressor," to quote Elie Wiesel. In this light, I would think that ethical relativism needs some qualification to it.