No word what exactly the problem is. Speculation are about that it has something to do with the Chinese take down of Twitter. There is no proof for that now.
According to their status-site it's "Currently experiencing elevated error rates", whatever that means. Ping times of twitter.com are extremely high up to 769ms. Did they "mess with the best" again :) ?
Update: Slow, but back online now. Still not clear what the problem was.