This would be really helpful for this very forum, where users could vote on suggestions and then you guys could see the most-requested features/issues. Not only that, but it would allow forum owners to create a Digg- or Reddit-type section in their community where users can vote on news/stories/threads/whatever. A simple sorting option would allow you to bring the most-voted threads to the top, etc.
I would think this would be relatively easy to implement, as simple as adding another field to the database table and writing the code to increment votes, as well as the required permission handling.
Also, this should be an option you can turn on or off forum-by-forum, not implemented globally.
It would also be nice if you could provide administrators the ability to turn downvotes on or off. I know a lot of people would want the ability to downvote a thread but the concept of a downvote has inherent negativity that I'm sure a lot of communities would want to avoid.