I still haven't been able to get on, though some reports have the site being back up. Early word is that Twitter was the victim of a denial of service attack, or DoS, which is when a Web site gets deliberately overloaded with huge numbers of computer-generated requests. (LAT)

Here's more from the WSJ:

While disruptive and hard to trace, this type of cyber attack is considered by experts to be a relatively unsophisticated technique. The attack itself doesn't attempt to infiltrate the internal operations of a company's computer infrastructure, simply rendering its Web site inactive. The attack on Twitter represents the latest in a string of denial-of-service attacks against several high-profile web sites. In early July, a number of U.S. government web sites, along with the web site of the New York Stock Exchange and several South Korean sites, suffered from similar actions.

*Finally back up but I'm still hearing it's hit and miss.