Fantastic article on fidelity in interface design:

The trick is to figure out which details help users identify the UI element, and which details distract from its intended meaning. Some details help users figure out what they’re looking at and how they can interact with it; other details distract from the idea you’re trying to convey. They turn your interface element from a concept into a specific thing. Thus, if an interface element is too distinct from its real-life counterpart, it becomes too hard to recognize. On the other hand, if it is too realistic, people are unable to figure out that you’re trying to communicate an idea, and what idea that might be.