The START bridging agreement will most likely include provisional implementation of the new treaty, rather than extension of the old one.
The bridging agreement that is supposed to close the gap between START and the START follow-on treaties created a bit of confusion in the last couple of days. It started from the remarks of NCS's Michael McFaul, who stated the obvious - the new treaty will not come into force by December 5th - and assured everybody that the U.S. and Russia are working on a "bridging agreement" that would make sure that some verification measures take place while the new treaty is going through ratification. For some reason some reporters took it to mean that the negotiators will not be completed in time.
I cannot find a direct quote, but it looks like McFaul said exactly the opposite - both sides are confident that the treaty will be signed and equally confident that it will not be ratified by December 5th. (Although, strictly speaking, the official line is that the treaty will be signed "by the end of the year".)
There was some confusion on the Russian side as well - the media quoted an official from the Information and Press Department of the Foreign Ministry official as saying that Russia could work on extending START to be that bridge. That was certainly not what the party line said - we know that there are many reasons why Russia would not want to extend START.
Today, a Foreign Ministry representative clarified the official position - Russia will be ready to start provisional application of the new treaty provisions before it enters into force. As I argued a few days ago, this is probably the best option at this point.