Twitter has been down for hours this morning; the site is defending against a denial of service attack. The Twitter Status blog says only:
"We are defending against a denial-of-service attack, and will update status again shortly."
An update says the site is back up, but at 11:25 a.m. it was down again.
Other reports say Facebook is also under attack.
UPDATE: Twitter is back. Here's PC World's take on why this outage mattered.
UPDATE 2: Today's attack was all aimed at one guy! More here.