I agree with Krystal, it's an interesting concept. I see two possible drawbacks. First, selecting the criteria (hours spent on line? Number of posts made? What would you use?). The second is the biggest one in my opinion and that would be time from the admins busy schedule to judge the criteria.

Both of these could be overcome, for instance if you use something simple like time on line it could even be automated with the computer posting the winner. I guess I would be in favor of it, but wish you LOTS of luck in getting admin interested in it.