I find the user experience of having to click the same arrow (up or down) to undo a vote counter-intuitive. I would expect to click the opposite arrow to undo a vote i.e. if I have up-voted I would expect to click the down arrow to undo my vote.
Maybe it's just me, but this gets me regularly.
Would it be possible (in addition to the existing undo behaviour) to make clicking down and up arrows decrement and increment by 1 respectively when a vote has already been cast. In this way I suspect undoing votes would work more intuitively for a bunch of users.