During my years of posting on various Western Religions boards, I have noticed that, in referring to atheists, some people use the term "disbelievers," some use "unbelievers," and still others use "non-believers." Are these synonymous, or is there a difference in meaning?

In this case I would say they are synonymous. You either believe in God (under any name) or you don't. You cannot "disbelieve" or "unbelieve" in the sense of reversing one's previous belief (as in "dismantle" or "unfasten"). For my money "non-believers" is technically the most accurate.