For a site as popular as Twitter, this is certainly a big shock to all the users, and probably an even bigger shock to Twitter themselves.
From the Twitter status update blog at 11:28pm on 12/18/09:
Update (11:28p): Twitter’s DNS records were temporarily compromised but have now been fixed. We are looking into the underlying cause and will update with more information soon.
And then another status update around 1:00am on 12/19/09:
We are working to recovery from an unplanned downtime and will update more as we learn the cause of this outage.
And like I said, not only did the hackers get into Twitter, they actually replaced the main site with their own image – and here it is: