Twitter is down, the company acknowledged 26 seconds ago, according to this screen capture from its status blog. (Hey, I could have been first to tweet it. Fail.)
I'm not sure how long it's been down, in part because when I went to check on downrightnow.com, downrightnow appeared to be down right now. (Update: Downrightnow is back now and reports the Twitter outage started a half-hour ago.)
These outages used to happen to Twitter so often that the "fail whale" became a meme. Now the company is grown up and the site reasonably reliable, so chances are they'll be back soon.
(Update: And they're back.)