According to CNN:

The Twitter outage began at 11:59 a.m. ET, according to Twitter’s page on tracking site Pingdom. Service returned intermittently around 1 p.m., but less than an hour later, Twitter crashed again.

“Engineers are currently working to resolve the issue,” a Twitter spokeswoman told CNNMoney.

Twitter updated its status blog at 1:42 saying “the issue has been resolved and all services are currently operational” — but at 2:16, another update from Twitter backtracked and said “the issue is on-going.”