According to the Twitter Status blog,the site was brought back up after an outage of a couple of hours, even as the company continued "defending against a denial-of-service attack."
Not clear whether the Livejournal outage at about the same was related.
Facebook traffic problem rumors were flying, too, but some felt that the social net's sluggishness was more a consequence of disenfranchised Tweeters flocking to Facebook to talk about their inability to Tweet than a result of an attack.
Which points out an unexpected and potentially problematic splash-factor when one of the top three or four services goes down and its hordes users overload other services: a new wrinkle in the distribution of DoS attacks.
Today's events may be a preview of a new wave of access attacks. The frustration-factor when an always (or nearly) on network such as Twitter goes dark, even briefly, is large. With tens of millions of Tweeters (and growing) the appeal of the target such a network presents is, alas, even larger.
To that end -- and not just to maintain your employees' ability to Tweet -- time to make sure none of your equipment is helping facilitate attacks.