Scott Sehlhorst has a "juicy" entry about the relationship among business rules, requirements, and design. He portrays the relationship as analogous to an onion. As you peel the onion, he contends, requirements lie out on the outside, followed by business rules, followed by design.
In discussing these relationships, Scott raises a number of thought-provoking issues and points, many of which I've addressed (as has Scott) in previous blog entries. I don't entirely agree with his main point about about requirements and business rules. I think the relationship is a bit more complex, as I mentioned here.