This is brought up so often it's unreal! I built this modification for the Doghouse forums and it works on threads as well as posts.
http://doghouse.forumdog.com/announcement.php?f=43&a=4
I am currently building it up into a publicly releasable version, because of the amount of times it's asked for and the personal requests I keep getting) which will also feature some niceties above and beyond the Doghouse version which was never intended for distribution.
I can't at this time say whether it will be freeware or paid-for. There is no ETA though I'm actively working on it right now and I don't have a complete feature-list.
The Facepunch rating is a simpler version is custom-made as far as I know, which you don't want anyway 'cos nobody wants to be like Facepunch. I did actually consider doing more than just +1 and -1 but it struck me during development that the whole beauty of the Digg method was it's absolute simplicity. It's just so ideal, hence I will not deviate from it.