I don't know how old the doctrine of rabbinic infallibility is, nor exactly how widespread it is. It certainly seems to have been with us for quite a while and also seems to be well entrenched in some of our communities. However, it's clearly not universally held. There are plenty of people in the Orthodox community who maintain that rabbanim can and do make mistakes. Even the gadol hador (no, not the blogger... we all *know* he's infallible. :) ) can make a mistake. This idea has clear support from the Torah, Navi, Mishna and the Gemara.
In any event, I was giving the matter of rabbinic infallibility some thought when I happened to listen to a lecture on the Kuzari Principle. The Kuzari Principle (in short, oversimplified form) states that the events of Mt. Sinai must be true because the entire nation witnessed it and passed the story of the Revelation to their children and grandchildren who, in turn, passed it along to their descendants, and so on. Had someone else invented the story of the Sinatic Revelation after the fact, the people would have rejected it saying "my father never told me that story." While it's certainly not a perfect proof to the factuality of the Sinatic Revelation, it's not a bad argument either.
However, it occurred to me that the doctrine of rabbinic infallibility actually wipes out the Kuzari Principle. If one accepts the premise that the leader or leaders of the generation are incapable of making an error, then it follows that they can formulate new traditions and have them be accepted by the masses. The fact that the audience did not receive these traditions from the ancestors won't matter. Just to take an extreme (and silly) example: Suppose a gadol announces that this year that everyone must, after eating the matzah at the seder, stand up and do the Macarena. He announces that he found sources which indicate that this practice is correct and should be done. Now, if you're going to posit that the rabbinic authority is always correct ("He's the gadol hador. Just listen to him. Be m'vatel your da'as to da'as torah.") then it does not matter that the people doing the Macarena this year never heard of this from their fathers or father's fathers -- they're going to take the gadol's word for it since he is infallible. But this scenario is exactly the sort of thing that undercuts the Kuzari Principle. The Kuzari Principle rests on the assumption that no one can introduce a new "history" to the nation because the check against that happening is the collective memory of the nation; but by granting someone the power to be infallible, you are giving them the ability to invent a new history and have it accepted despite the collective memory of the population saying otherwise. Therefore, in any system where the Kuzari Prinicple is accepted, human infallibility cannot exist.