The reason people say it's a countdown to a new Diablo book, is because when you cick the Blizzplanet countdown link, it links to the front page of this site, and the first article you see is about a book. Umaro or MD you need to fix that!
I doubt this will be about a patch for Diablo 2.. I mean, will diablo3.com get a spectacular increase in visitors because of a patch for Diablo 2? I doubt that.
My vote is for something regarding Diablo 3, either some subtle hint from Blizzard or a countdown will start in 13 days on Blizzard's website just like they did with Starcraft 2. Might also just be wishful thinking on my part though