Twitter is down because it is experience DDoS attack for more than 2 hours now. See http://status.twitter.com/
I have posted an article yesterday on blog on DDoS. This is almost impossible to control ebay, yahoo, amazon all have faced this DDoS attack. History of DDoS attack goes back to 2000.
Interestingly, according to BBC yesterday's DDOS attacks were aimed at a single individual. A blogger called Cyxymu. Read the news story here.
its not just twitter http://www.thinkdigit.com/Internet/Twitter-Facebook-go-down-emptiness-of-lives_3278.html