Maybe it's been there for a couple days but this is the first time I've noticed it. It seems cool, but it makes the news feed look even more cluttered and I worry about what they'll do with my data--will one of my friends find out that I voted their story down?
Unfortunately there's no "vote down" button for the ads... you can downvote Facebook Pages news items though.
It doesn't really bother me, but my news feed is already pretty relevant because I adjusted http://pitt.facebook.com/feed_prefs.php. I have a feeling that most people have never clicked on the "Preferences" button at the top of the news feed.