Is Twitter down?

I’m having problems accessing Twitter and my third-party clients can’t access Twitter’s API as well.  And it turn’s out I’m not the only one that can’t get access as well.  My first reaction was that Siggraph blocked Twitter but I kept getting timeout responses on the AT&T network as well.  So the real question is: which famous celebrity died?

UPDATE: Twitter has confirmed that the site is down due to denial-of-service attack.  In a blog posting, the site is “defending against a denial-of-service attack, and will update status again shortly.”