That's a tough call though; some people might argue about what being an 'active poster' means, someone might have 100 posts and call themselves active, yet someone with 1,000 posts could do the same. You need to set boundries first.
Good idea overall though, an incentive to keep posting.
i think that this is kind of like forcing people to post because they get something out of it. a good forum will plenty of stuff to talk about should not have to reword people for posting. this should happen naturally. but that's how I see it.