It's good to note that while theme code is GPL, the CSS and Images are not necessarily so. Its the exception to the copy-left, and probably the only way to protect "premium" themes (besides using web-services on a 3rd party server, which is more likely in plugins). This means while you can take code (unethical?) from premium themes, you might not (depending on the license) be able to "borrow" the css and graphics.

The rule of thumb is if a look "feels" enough like the original where a user can confuse the branding, you've infringed. Trademarks and copyrights are usually dependent on a market, so using golden arches for a burger joint is a no-no, but for a computer shop called Mac Mechanic, maybe its kool ;-)