Around 1opm PST Twitter went down and it went down HARD !!
All Twitter subdomains are down and compromised.
I'm in agreement with the folks at TechCrunch, this has the potential to be a significant breach (we'll find out from the press release after they get the site back up), but you should probably go through and change your password when access is granted again.
Below is the message that was displayed on the twitter.com site from those claiming responsibility for the hack: