Actually.. This happens all the time. In fact, the brushed metal appearance, by Apple's own guidelines, is not supposed to be used on software like browsers. Yet they happily violated their own rules.

Not that I have a problem with that! Rules are not meant to be followed to the letter. They are called 'guidlines' because they are there to 'guide' development, not force it. Start with the rules, and then if you find a need, then start to stray from them until you find a good solution. I'll agree tabs aren't perfect, but not because they violate any guidelines. I find them pretty intuitive to use. I hope safari gets tabs, or some similar mechanism for organizing windows. I think its clear that the individual window for each webpage metaphor isn't very effective given most people's browsing habits. "it violates the rules" is no good argument for how to design software.

When people have the sense to know when to start violating their own rules, thats when real innovation happens.