I think a ranking would not be difficult to implement.
In fact, I have already seen something similar in action on Damian's personal page, I believe... just went over there to check, but it looks like something else: he shows the top posters, not the top scorers.
Your rank: (based on current workunits received) Your rank out of 5064232 total users is: 80444th place. The number of users who have this rank: 25 You have completed more work units than 98.411% of our users.
How difficult can it be to do the same based on the scoring system?