Cultural relativism represents an important and necessary critique of modernity, but when it is expanded into the stand alone philosophy of ethical relativism it is hogwash.
Rather than pick a favored definition, I'll link to the range given by Google.
Cultural relativism: http://www.google.com/search?hl=en&lr=&oi=defmore&q=define:Cultural+relativism
Ethical relativism: http://www.google.com/search?hl=en&lr=&q=define%3Aethical+relativism
The first critique I'll make is that it contains an internal contradiction large enough to drive a mobile home through. Namely, many cultures have universal aspects; typically these aspects are ideological or religious. For example: many Christians believe they need to save the souls of heathens, Communists tend to want the workers of the world to unite, classical Liberals want rights for everyone. These impulses are both genuinely part of cultures, but in direct contradiction to the values of ethical relativism.
This contradiction is profound, it shows that culture and society can be wrong under an ethical relativism definition.
"So," you say, "Big deal, there's an internal contradiction. Most philosophies have some, what's the problem?" The problem isn't really the contradiction, but the underlying assumption that the contradiction exposes. Namely, to an ethical relativist there may be no universal laws, but there is a highest virtue: not judging others.
Now, there's some merit in this virtue. People have certainly made a lot of stupid judgments over the years and have caused a lot of pain as a result. Moreover some belief systems, like Christianity, have even given an explicit shout-out to this virtue. However, when I say someone has good judgment it's a compliment. Being a judge is a position of honor, not one of shame. How can this be? Does it show that I and our society is nuts?
Well, both may be the case, but so is ethical relativism. I do believe that not being highly judgmental can be a virtue, particularly if judgment is applied without first understanding individual and cultural context. However, it is by no means the highest virtue. Promoting the capability to experience life, liberty, and the pursuit of happiness are three separate virtues that rank far above not being judgment in my book. For example, the context of amalthya's quote was that America today is better than America in 1850. We have cable, they had slavery, cable is much better than slavery.
Ethical relativism approaches moral discussion by saying "I'm taking my toys and going home." It recognizes a truth modernity rejects, that all this is horribly complex and what works for one culture or individual might not work for another culture or individual. However, this realization does not mean we must end debate, just that we must look deeper and be willing to face the dark elements of our own societies.
Are all moral questions solvable? No, many questions do turn on genuinely subjective calls. There is still a place for debating subjective views and the beliefs behind them, but relativism is correct in proposing that we should treat the views of others on subjective matters with great respect. Nonetheless, murder, slavery, genocide, torture, and mutilation are all evil. I do argue that evil at times may be necessary, but it doesn't stop being evil.
So by all means question the assumptions of modernity. But when you do so, be sure to apply your judgment.