IBM tumbles as Buffett discloses he is trimming stake

IBM shares tumbled Friday after billionaire investor Warren Buffett revealed he has sold about a third of his stake and "revalued" downward the computing giant.In early Wall Street trade, IBM shares were down 2.5 percent at $154.00.Buffett, who heads the Berkshire Hathaway holding company and is among the world's richest individuals, invested some $10.7 billion in IBM in 2011 for a stake of some 5.5 percent.