This must already sort-of exist: check out pg himself (http://arclanguage.org/user?id=pg). He's got 1190 karma, whereas almkglor, the current leader, has "only" 660. So somehow, pg's opting out of the "leaderboard" system, at least. If that's the case, it should be in news.arc somewhere (I'll look if I get the chance); of course, there could be some other reason I'm missing.