Nice article on Smashing Magazine by Kate England. It’s the Human Interface Guidelines for icons in a nutshell: Mac OS X Leopard: Designer’s Guide to Icons:
To get really beautiful icons, Apple recommends that you let a professional designer create your icons. Despite all the eye candy and realism that is possible to apply to icons, less is more. Strive for a simple solution using one easily recognisable object. The basic shape or silhouette of your icon can help users to quickly identify it. If you aim for an international market, your symbols need to also be internationally recognizable providing Worldwide Compatibility.