I am of the belief that to leave this kind of cliff hanger is beneath an accomplished writer. It is usually a cheap mechanism used by inferior authors to get readers to buy the next book, and I think it was a little beneath GRRM to employ a cliff-hanger ending, unless he has tWoW in the can and ready to go for Christmas 2012 (which we know he doesn't). He knows that we love his books, and it is kind of cruel to inflict this level of pain on us. I mean, it is not as if it's Brienne's so-called last word (we're still waiting on that one!), but the death/un-death of a major character who appeared in the first chapters of the first book and has been with us ever since.
He could have completed Jon's story in ADWD by having a final chapter, where he wakes up in the ice cell, as is speculated above, and then we are left to speculate on his response to the treachery of his so-called brothers. Alas, he did not, and I find his use of this major cliffhanger disappointing, to say the least, and cheap, particularly given the long waits between books.
Anyway, I really hope Jon's not dead.
(murderous GRRM b*****d!)