Amazon shares jump as it joins S&P 500

CNET News.com: "Standard & Poor's on Monday said Amazon.com will join its flagship 500 stock index after the close of trading on Nov. 18, replacing AT&T.
The news sent the Internet retailer's shares up 6 percent in after-hours trading.
Share prices of companies joining the S&P 500 often rise because many portfolio managers try to track the index, and they are required by charter to buy stocks that enter it.
Seattle-based Amazon, which began as an online bookseller, has grown to become the second most popular Internet commerce site, after eBay. "