For some reason I get the urge to kick up this testy old thread with a new spin on the issue:
Psychological and neuroscience research seems to support the idea that humans are inherently and naturally good. (It's not proven yet - not that any theory is ever really "proven" - but there is some substantial support for the idea. Type "self-determination theory into Google Scholar for a starting point.) Basically, the theory is that if a person is mentally healthy and has grown up and lives in a healthy environment, then he or she is likely to engage in a lifestyle that fits in with what are generally considered good morals. It's a powerful theory that has explanatory and predictive power for all sorts of situations, and it is my unprofessional opinion that future research will help us understand this issue in more detail while still generally supporting the idea that people are inherently good.
So, to me, if someone is religious, often they will effectively be putting the word "God" where I believe the words "healthy intrinsic motivation" are more appropriate. For example, if someone is going through some hard times, he or she might try to hold onto the idea of "being guided by God" or something like that as a way of trying to stick to good morals - self determination theory would say that the person is struggling to keep his or her healthy intrinsic motivation intact in spite of some factors that are compromising his or her core psychological needs for autonomy, competence, and relatedness.
Bottom line, science seems to indicate that people are inherently good.