That is a real tweet sent out by the real Associated Press account, but the AP didn't write it or send it. It seems the account was hacked by a group calling themselves the Syrian Electronic Army.
It took about 30 seconds for savvy Twitter users to figure out the hack, comparing style and syntax to normal AP tweets, but the markets had already reacted.
The AP says their accounts had been the targets of repeated phishing attempts over the past few days.
The false tweet said there had been two explosions at the White House and that President Barack Obama was injured. The attack on AP's Twitter account and the AP Mobile Twitter account was preceded by phishing attempts on AP's corporate network.
Market loss in that short 100-point dip was estimated to be $100 billion or so. They bounced right back as soon as the tweet was confirmed to be the result of a hack, but it certainly highlights how real time information flows can be used to game the markets. I'll be interested to discover whether anyone profited from that brief dip in stock values.