so nobody's going to sue you for snitching a few design points. Text, graphics, and images are a different thing however, and somebody (if not the site owner) is going to have copyright on those.
What you might be guilty of, if you slavishly duplicate the look of, say, the BBC website is what's known as 'passing off', which is what happens when someone deliberately makes product B (which has no reputation in the market) look exactly like Product A (which is a market leader). Passing off is different to forgery - if I wanted to pass off an inferior web forum as this one I might duplicate the look, but call it 'PC Advertisor' - using the same typeface. A forgery however, would duplicate the name exactly.
That's a very much simplified explanation of some complex legal issues, but suffice it to say you will be OK if you copy the odd code snippet and design idea. Avoid copying entire websites though.