I've added a similar function to my CMS mod that I'll release soon, and in commenting system I've this Like/Dislike function, which I plan to seperate it from my main mod and post for free as a post like/dislike addon. So if you can wait till the end of September you can get it.
PS: You can see how it will likes at:
http://www.christeris.net/vb42/comments.php Just keep in mind that in my commenting section I don't need to show the usernames something that will exists in the seperate addon
Chris