There are downvotes on HN, you just need to reach some karma (500 if I'm correct) before being able to do so.
That is for comments, he said posts. You cannot vote down posts (submissions) at all. I'm obviously aware of the downvote button on comments, and thanks to those of you who tried to prove a point by hitting yours on this one.
Once one has more than 500 karma one gets a downvote button..