More info http://www.theverge.com/2012/12/3/3722112/tumblr-hack-gnaa

Tumblr are suggesting you log out/clear cookies if you visited any of the hacked sites

> There is a viral post circulating on Tumblr which begins "Dearest 'Tumblr' users". If you have viewed this post, please log out of all browsers that may be using Tumblr immediately. Our engineers are working to resolve the issue as swiftly as possible. Thank you.