Apple becomes biggest company in U.S. history; stock reaches $623B

Apple has become the most valuable company in American history. Its surging stock propelled the company's value to $623 billion, beating the record for market capitalization set by Microsoft Corp. in the heady days of the Internet boom.

Apple's stock has hit new highs recently because of optimism around what is believed to be the impending launch of the iPhone 5, and possibly a smaller, cheaper iPad.

The company is now worth 53 percent more than No. 2 Exxon Mobil Corp. Microsoft is worth $257 billion.