I've known for awhile, through the magical publishing grapevine (it does not come with singing raisins) that the last Wheel of Time book was going to be split. I didn't know why; I just knew it was running long and that was that.
And now there's apparently a press release out about it, and Brandon Sanderson has written a wonderful blog post about why it takes so darn long to write a big book. Go read it. It's good.