Of note/concern: Early this morning, we spotted a Tumblr user who was unknowingly having spyware injected onto their reblogs. We could tell this because of code added at the bottom of their pages. No word of how widespread this is, but doing a search shows that multiple Tumblrs are affected by the code injection. Suffice it to say: Be careful what you reblog.
EDIT: Some advice on how to avoid the hijack.