It keeps on changing all the time. I don't think anybody here can tell you the answer except maybe the guys at big "G"
I think they are changing quite often, I would say every PR update. they always come up witht he new ideas to let us confused about bulding PR. lol
Seems to me last 3-5 months were full of changes. Maybe they are preparing Algos for the new DIGG/G search engine approaches?
I've noticed gradual changes. They now put Thumbnails beside links that leads to videos, and they put Author names beside a article. (If the article is not spam)
I think they are constantly changing it, but not dramatically. Small steps have been taken this year to devalue a bunch of links such as paid links, directory links and the like.
they roll out about on average 9 algo tweaks / mods per week and rebuild and recalculate the link matrix almost daily now. there is no such thing as a stable serp or rankings any more