According to the Bantam rep at Comicon (I wasn't there - wrong side of the Atlantic - but know someone who was), George's editors were 'cracking the whip' for him to get the book done by the end of June. That's because if he doesn't finish it then, then it'll stay unfinished until after Worldcon in early August (since he's going to Europe in the first week of July, IIRC). Even if he finished it by the end of August, the chances of it making it out even in the UK before the end of 2008 become somewhere close to zero.
The question is whether he still has new material to write, or if he has actually finished the book in draft form and is now revising the last few chapters to bring them up to speed. If so, it's possible that the publishers will just say to hell with it and publish with what they've got by the end of June. I get the impression Bantam know that there will be a dearth of big fantasy and particularly big epic fantasy books coming out at the end of 2008 (Pratchett's Nation in the only other one that comes immediately to mind, perhaps Gaiman's new book as well) and if they get it out by then, the book could do much better than if it's running headlong into the new Lynch and particularly the new Rothfuss and Jordan next year.