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