I agree with Nornagest that this would be really annoying for many non-abusive posters.

I'm also not sure how effective it would be against abusive mass down-voters. Whoever's currently stalking me is systematically downvoting a bunch of my old comments every day. I assume s/he is just gradually going through LW's list of my comments. It doesn't seem like it would be difficult to do it gradually throughout the day, or even to write a computer program to do it automatically.