While I could like to all sorts of technical websites, the guys who run Car Talk, a popular North American radio show about cars, do a better job of descibing the situation. Read about it here. My favourite quote:
We don't believe that any modern engine that claims to require premium will be damaged by using regular unleaded judiciously. Neither do any of the sources we've checked with — including the American Petroleum Institute, the American Engine Rebuilders Association — even a chemist (who would rather go unnamed) at a major gasoline company.
So save some money and use regular gas!