No. pg has posted about this several times. I can't find one of those comments with a casual search, but it's pretty close to: start with the most recent 50 comments you've made, throw out the highest-scoring one, average the remaining ones. The focus is to average over your recent activity.
Just an empirical observation, your average karma seems to recalculate on a periodic as opposed to a continual basis. I've noticed at times that, after posting something that gets a lot of upvotes, checking my profile (even when I don't hit my browser cache) doesn't show any increase to my average, yet waiting a day or so does.
This is purely anecdotal and unscientific, but it's a consistent observation.
