Explained by Matt Cutts: http://www.mattcutts.com/blog/pagerank-sculpting/ I think because of this move most of the sites will lose Page Ranks.
The PageRank algorithm is changing time to time or we say it keep developing. So, no one actually know the exactly how it works except google.
mine's only been going one way since last 2 updates.. upwards... don't see it coming down anyway though.
If you take a look at the events concerning PR since last year up to now, you will notice that Google regularly has been working on measures to evaluating the links effectiveness in a harder way and so more sites are losing PR.
Well take heart that even if your lose PR it's not the end of the world. Search engine ranking is more important. PR only accounts 20% of how much it influences your search engine ranking. Peter Lee