At what magical point in the future will scholars suddenly agree that a whole host of new constructions shall be granted acceptability from on high?
I'll stop being flippant now. I have long been interested in this topic, although my lack of confidence in my own grammer has stopped me from posting before.

My background is a little different from DMW's. I debated in High School and College and am now in Grad School studying Rhetoric with a focus on social movements; I am a speaker, not a writer, and therefore have always been more interested if something "sounds right" than if something "is correct."
It seems that everyone will agree that language evolves, and that grammer is necessary. But to reduce the discussion of usage to just these issues is to oversimplify.
I would argue that whether 'his' is gendered is a fundamentally political question. I would wager that many, many people who use 'their' know full well what they are doing, and it has nothing to do with the debate of how language evolves or the integrity of grammer. To divorce the subject of usage from the social and political nature of our being is a diservice to anyone who loves the language as much as all of us here apparently do. Deliberatly using incorrect grammer as an act of resistance has a great and fascinating history.
I am thinking of children from the South (or parts of London, or anywhere, for that matter) for whom going to school was a torture because all they had ever heard before was "wrong." I know that this is an issue far greater than grammer, but they are internally related in such a way that I don't think the political can be ignored.
So, DMW, I would certainly agree that you should teach students the standards of usage. But I don't think it would hurt a bit to introduce a hint of ambiguity . The idea that people can argue, debate, write dissertations, scream, fight, and go on and on for days on end about a subject so supposedly "dry" as grammer would be a very great lesson indeed.
(My grammer check would go nuts over this last sentance, but orally it sings like Cicero
