According to Twitter's status page, as of about 10:50 am ET, "We are defending against a denial-of-service attack, and will update status again shortly." Twitter adds, "we are defending against a denial-of-service attack." Argh, how can I live without this social utility I didn't even know existed a year ago and which I thought was stupid as late as 7 or 8 months ago? :)
UPDATE 11:16 am ET: "The site is back up, but we are continuing to defend against and recover from this attack."
UPDATE 11:53 am ET: The New York Times reports, "Facebook also seemed to be experiencing problems, at least intermittently."