I think everybody would agree at this point that the odds that the START follow-on treaty will be signed before START expires on Saturday are not particularly high. Unless, of course, the presidents are willing to dash to Reykjavik or Geneva for a quick signing ceremony on an extremely short notice. I would not discount this possibility entirely - the symbolic value of not having a gap between arms control agreements is fairly high - but the signs are not very encouraging.
On some level it wouldn't really matter - the new treaty had no chance of entering into force before the deadline anyway. But it would be a serious blow to the disarmament efforts. A failure would make a bridging transparency arrangement very much impossible. But more importantly, if there are issues that cannot be resolved by December 5th, I don't think they will be resolved by December 25th (or by May 2010, for that matter).