Hmm.... never crossed my mind. There seems to be a pretty heavy lurker community on my board, not sure how large though. New members are constantly saying "I've been visiting for a while, just not posting..."
I think the ratio of lurkers to posters depends quite a bit on the overall purpose of the forum as well. In my forums, the purpose is to provide a place to share stories -- so having one post and five hundred read (or lurk) is perfectly all right for me!