PR Update?

Well... I am just really confused now! My friend told me that Google updates PageRanks every three or four months and I believed her because I've also heard the same fact from my other blogger friends and on some PR articles.

Now, I guess December 2008 is the third month from their previous PR update. And now, I just want to ask you guys if Google updated PRs this month? Did they?

My one friend lost his PR one month after the previous update. My other friend lost her PR last week. I really don't understand. Hehehe. My blogs did not change their PRs. Hahaha. Can anyone explain? Did Google update PRs already for the month of December? Thanks.