Here is some interesting data from Christian pollster George Barna:

...A large majority of evangelicals (70%) report believing that moral truth is absolute. But a minority of non-evangelical born again adults (42%) holds that same view, and even fewer of the notional Christians (25%), people associated with non-Christian faiths (16%) and those who claim to be atheist or agnostic (27%) embrace moral absolutes.

So what's up with "atheist and agnostic" people being more absolutest than those who are "non-Christian." Barna's standards for "Christianity" are pretty high, and "notional Christians" would probably fall under the rubric of mainline Protestants and other liberal and moderate believers who reject literalism and are not "Born Again." When one thinks of non-Christian faiths one assumes Jews, Muslims, Hindus, etc., but I suspect Barna has slotted the large number of Americans who are New Age and "Spiritual," who claim no specific religion but believe in God, into this group. And this explains the rejection of moral absolutes. Atheists and agnostics might be relativists, but for the term "atheist" to be intelligible my impression is that you need to have a traditional Enlightenment view of objective reality and truths, which I think is why they are less likely to reject moral absolutes (they simply don't accept conservative Christian moral absolutes, they often have their own alternatives).