Bantam are now listing The World of Ice and Fire for publication on 30 October 2012.
For those not in the know, this is a companion to the SoIaF novels (but not the TV series, which is getting its own, separate companion/making-of book in 2012), co-written by George R.R. Martin with Elio Garcia and Linda Antonsson, the admins of the Westeros.org website (the site GRRM himself uses to double-check details if he's in a hurry). It's a large-format hardcover which will feature extensive artwork, including castle pictures by noted Tolkien artist Ted Nasmith, a number of character portraits and some all-new maps. To be apparently included will be maps of the never-before-seen Jade Sea region, and a 'world map' showing all the lands featured in the books (ranging from Westeros to Qarth). There will also be an extensive history of the world written from the POV of a maester of the Citadel.
I'm waiting for confirmation from Bantam that this release date is locked, but interesting news if so. The book has been four years in the making, and should be an interesting read.