As much as possible, allow users to do whatever they want at all times. Avoid using modes that lock them into one operation and prevent them from working on anything else until that operation is completed. -- Apple Human Interface Guidelines
Dear Apple iLife team. Please remove the distinction between "browse" and "edit" modes in iPhoto. It is the perfect example of why modal interfaces are bad, as I constantly find myself in the wrong mode for whatever operation I happen to want to perform.