Chairman and CEO, Berkshire Hathaway
Of course it matters that Buffett has built Berkshire Hathaway into a massive holding company with interests ranging from underwear to private jets (2006 revenues: $98 billion). Of course it's impressive that since 1965, Berkshire has performed more than twice as well as the S&P 500. Of course it's amazing that Buffett has made millions from something as toxic as Enron bonds. And of course it is somehow unsurprising that he managed to help broker a deal between A-Rod and the New York Yankees.
But the source of Buffett's influence is found in his nickname: the "Sage of Omaha." CEOs - and athletes, obviously, including LeBron James - venture to Nebraska to consult him. And people everywhere listen to his pronouncements on things like managed earnings (against), stock option expensing (for), and the U.S. dollar (pessimistic).