From the official Twitter blog...

Man down!
Around 11 am in San Francisco, our main database db006, crashed because
 of too many connections.We have to put the service into an unscheduled
 maintenance mode to recover.Folks will see degraded service
 for the next few hours.

We'll keep you updated here and in Satisfaction.

....Sorry, Just had some major Deja Vu there, It's understandable that a rush of visitors can cause this, They're not psychic, But for this to follow after weeks of unstable service, and downtime, makes you wonder what their petty cash is ending up on!