I think it was Ravyn who mentioned an on the need to know basis. That's pretty much how I approach it. If people ask, I will most likely tell them, but it really depends on the situation.
I'm pretty much the same way, but I feel more comfortable with my beliefs now so if religion comes up in conversation with people I know well I will likely mention my background. Plus, I work with a bunch of Biologists (being a biology grad. student myself), which seem to be a pretty religiously tolerant group. At least if you aren't a fundamentalist.