Twitter was offline for an extended period of time on August 6th 2009 due to a DDoS ( massive distributed denial-of-service attacks ). Twitter left a message during the attack reporting it's outage: "We are defending against a denial-of-service attack, and will update status again shortly."
The attack is linked to the launch of a new Koobface malware run which used Twitter as a distribution vector. The malware is distributed to victims via fake Facebook pages and varying Twitter messages that link to scareware pages (fake security).
Twitter began to use Google's Safe Browsing API to filter out bad Twitter links. The Koobface links however are bypassing the Google Safe Browsing API.