Apparently, the manuscript got so long, he and his publisher just decided to split the book, move some of it into the next book (A DANCE WITH DRAGONS), and get FEAST out ASAP.
I’m really looking forward to this. If you haven’t read the first three books (A GAME OF THRONES, A STORM OF SWORDS, and A CLASH OF KINGS) do so, especially if you have a cynical view of huge-volume fantasy series. This one started out looking like a fairly straightforward fantasy take on the War of the Roses, but has really impressed me. I highly recommend them.