I have long supported straight extension of the START Treaty - mostly because it provides the transparency and verification framework that we know will not be replicated in the the follow-on agreement. But we are where we are - I think the chances of START being extended are exactly zero. Sadly, this is because Russia made very much everything it can to make sure that this option is not on the table (RS-24 was the "poison pill" that very much was designed to do that).
Now that the end of START is near, the idea of extending the treaty (or at least its verification procedures) is getting popular in Congress, with the Republicans emerging as the main proponents of the measure. One of ther main arguments is that the extension would require no legislative approval and therefore can be done very easily. While this is true on the U.S. side, in Russia the situation is not as clear (is it ever?).
The law (that would be the Federal Law on International Treaties of 2003) says that any treaty or agreement that deals with arms control or disarmament has to be ratified by the Duma (Article 15.1g). The formal purpose of ratification is to incorporate the treaty into the legal system - in Russia, international obligations supersede national laws.
A good lawyer would probably argue that a simple extension does not create any new obligations, so ratification is not necessary. I think this argument has merit, but the established practice disagrees - the Duma has been routinely ratifying all kind of protocols that extend previously signed agreements.
There is still some room for a legal argument, I believe - unlike many of those agreements, START Treaty has the procedure embedded in the treaty, so its extension may not, in fact, require a protocol. For example, I see no reason why it cannot be done by a joint presidential statement. That would certainly not require ratification.
The real problem, of course, is not in the legalities of the procedure - if there were the political will, START would have been extended without ratification or with a very quick approval by the Duma. The problem is that no one in Russia seems to favor the START extension (I may well be the only Russian who does, but I'm not there). Theoretically, President Medvedev could sign a joint statement and then push it through the political system. But in practice he doesn't seem to have the weight to do that. Besides, why would he? He would much rather make the new treaty his signature cause.
I think we should just accept that START will expire and cast our lot with the new treaty. I agree with those who believe that the verification gap that will open on December 5th is something to worry about, but we have a way of dealing with that - by provisional application of the new treaty requirements.