A lot of people whose karma otherwise rate leaderboard status have been manually removed (especially pg).
In nickb's case, it's likely because his account has been dormant for almost a year. He may have passed away (http://markenomics.com/item?id=395) but it's also likely he just decided to take his leave of Hacker News and a number of other online communities.
And where the heck is nickb? He also disappeared off the leader board, so I have my suspicions. :) hrm.