Luc,
I often post on the tek-tips forums. They have a rather great system of marking post as useful. Marking a post (not a topic) as useful has 2 nice effects. 1. The topic COULD be higher in the search results and the person who posted this replay is rewarded with (I think) points.
The top posters list is then NOT a list of people posting the most on the forum, but a list of people posting the best answers. This is way this list is called the forum experts.
Maybe a list of forum experts makes the answers meot useful, but more important it's far more easier to find a good (and good rated) answer to a question.
I think this will decrease the searching-the-forum-before-posting problem and increase the search functionality of the forum.
But, it should be possible to rate a post at any time by anyone, and people should offcourse rate a post.
Hope this functionality could be implemented, or even part of it
some usefull links maybe:
http://www.phpbb.com/phpBB/viewtopic.php?t=119205