Apple has acknowledged that demand for iPhones is waning. That confirms investor fears that the company's most profitable product has lost some of its lustre. Apple CEO Tim Cook sent a letter to the company's shareholders, released after the stock market closed Wednesday. Cook said Apple's revenue for the October-December quarter will fall well below the company's earlier projections. That period includes the crucial holiday shopping season. Apple now expects revenue of $84 billion for the period. Official results are scheduled to be released Jan. 29. Cook traced most of the revenue drop to China, where the economy has been slowing and Apple has faced tougher competition.