The Twitter social networking site is experiencing a denial-of-service attack, which took the site down for a while starting around 9 AM EDT this morning. Further information is available at the Twitter status page. Another social networking site, Facebook, seems to be having problems this morning also, although on a somewhat intermittent basis. Not much more information is available at present.
Update Thursday, August 6, 17:45
Apparently both Twitter and Facebook were the targets of DDoS attacks. The “Epicenter” blog at Wired has a post with many of the details.