Twitter was hit with a denial of service attack this morning starting at 9:30am. The site was down for about 2 hours before it was brought back online. At this time, it is still being hit with the attack but is able to stay online. A message from Twitter was released stating:

"We are continuing to defend against and recover from this attack."

A denial of service attack is typically an overwhelming flood of traffic that is trying to bring down a site. In this case maybe someone just tried to send a message that was 141 characters. Maybe Twitter didn't think of that.