So George RR Martin posted on his livejournal blog that he is skipping a few cons this year, namely the World Fantasy Convention in Saratoga, and the San Diego Comic Con. That is unless he "should complete and deliver WINDS OF WINTER before these cons roll around."
Now, he has never before commented on his progress on his books, notably saying 'it'll be finished when it's finished' and nothing more than that. His publishers earlier this year stated they don't have it on a list of release for 2015. We have no concrete date. What we have an offhand comment that he made, that very well might be a joke.
However many sites are using this single sentence as confirmation that he could be done writing as early as July (before San Diego Comic Con) which I think is a stretch, or at least by the end of this year. And there are still more people claiming we could have a published book by the end of this year.
So expect to hear the internet use this to continue to say that we're really close to it's release at any time now, and take these kinds of rumours with a grain of salt until we have something more concrete.
Honestly, I kind of wish he would go to the Cons because if all he is doing is sitting and writing and not doing the things he likes then that might impact the quality of the book, and I'm more for quality than "give it to me now!"