I find it interesting how many people here (including myself) assumed you literally voted it both up and down. I rather liked the idea myself, since I hadn't even considered that set of actions.

I'm also curious now, whether that action would be functionally different from abstaining. I'd assume it eats one point of your "downvote capacity" and nothing more, but I could see a system where comments get flagged as "controversial" due to lots of votes in both directions (I even recall a "controversial" flag in the code somewhere...)