We are aware that there is a viral post circulating on Tumblr. We are working to resolve the issue as swiftly as possible.Thank you.
— Tumblr (@tumblr) December 3, 2012
Following the infection of many Tumblr sites with a worm today, Tumblr have temporarity suspended posting.
Update: Technical detail on how the worm spread are being revealed
You can read more about the attack on Sophos Naked Security