"The Twitter website is being widely exploited by users who have stumbled across a flaw which allows messages to pop-up and third-party websites to open in your browser just by moving your mouse over a link. Messages are also spreading virally exploiting the vulnerability without the consent of users."
As of 9:20AM EST, I have also seen attacks doing the same thing using an overlay, making simply viewing your account reproduce the offending messages. PC Magazine's Larry Seltzer's blog claims the attacks were stopped around 9AM but began again around 9:15. Perhaps the original fix only blocked onmouseover.