pg mentioned it once but I can't find it. It's some combination of votes and comments, with an exponential decay function to lower it over time. So if gazillions of people vote it up, it can stay on the front for a while, or if several people start commenting on a new article, it can jump to the front page even with only a few points.

Incidentally, this means that all of the people who complain about xkcd comics are ensuring that it gets on the front page, where more people will look at it, vote for it, and comment on it, prolonging its time on the front page!

As of a few days ago, you can just RTFS :-)

  (= gravity* 1.4 timebase* 120 front-threshold* 1)

  (def frontpage-rank (s (o gravity gravity*))
    (/ (- (realscore s) 1)
       (expt (/ (+ (item-age s) timebase*) 60) gravity)))