I want to begin by acknowledging the fact that correcting other people's grammar is an annoying habit. And it's a habit that I definitely possess, as Ian can attest. He has to live with me harping on him for saying things like "nucular" instead of "nuclear" and "I should have went" instead of "I should have gone". I know, I'm a harpy. At least I am aware of it. And no, I'm sorry, I don't think this is something I can change.
But I do think there are limits to the acceptability of correcting people's grammar. For instance, you simply don't correct the grammar of strangers. Like you don't call a secretary that you don't know personally, and tell her that it's "out for lunch" and not "out to lunch", because if you do, you are a rude asshole who also happens to be wrong.
You also don't belittle an emotional gesture by focusing on grammar. For instance, I once read in the advice columns that I so lovingly read that a woman was upset when her daughter wrote a thank you letter to her grandmother, and it was returned all marked up in red. Can we say bad taste? Yeah, it's best to just focus on the gesture and not cheapen it by going grammatical nazi on someone, even in the pretense of "teaching".
This also happened to me when I was having the break-up talk with Like Me But Not in Love With Me. I brought a letter laying out everything that I wanted to say to him, because I wanted to make sure I remembered to say it all in the heat of the emotional break-up. I mistakenly wrote "you" when I should have written "your", and that's the first thing he said to me when he popped his head up after reading my letter. And it wasn't even a mean letter but a sincerely heartfelt letter. His remarking on the grammar, though perhaps a defense mechanism against emotion, completely cheapened the emotional expression I had been trying to make.
The fine line here is that there is context within which it is ok (if still annoying) to correct grammar, and there are times when it is definitely not ok. I like to think that I do a good job of respecting those boundaries, but people like Ian who receive my wrath might tell you that a few more boundaries ought to be drawn. Eh, I can think of worse habits that Ian might have to live with, so to him I say consider thyself blessed.
I like to think that I strike a decent balance between welcoming linguistic change and upholding linguistic tradition. Officially, I am a descriptivist, which means I am more prone to say that usage determines correctness, and if enough people use a certain form, then it is correct. Language, after all, is ever-changing. Things that used to be prevalent, like say double or even triple negatives (which were used to add emphasis), are no longer considered acceptable, but maybe one day they shall return. Change is the norm in grammar. Having said that, there lives inside me a nasty little grammatical prescriptivist who slaps down her ruler on the hands of people who, for instance, fail to understand the proper distinction between who vs. whom. This is where my wrath is taking us today.
First of all, I totally recognize that this grammatical rule is not properly explained in English classes (if indeed any grammatical rule is properly explained in English, aka literature, classes). I was fortunate enough to have an English teacher who knew a very easy trick for deciding whether you should use who or whom. She said, "If you can replace who/whom with him/her/them, then whom is the proper choice, if you can replace it with he/she/it, then who is proper." This is easy to remember because whom, him, and them all end with "m".
But most people don't even know why we have a who and a whom in the first place. Do you know why? If not, don't feel bad, I'm about to tell you. Simply put, there are two main functions that a noun or pronoun can play in a sentence - a subject function, or an object function. The subject does the action of the sentence, and the object receives the action of the sentence. (And the action is the verb, if you were wondering.) Take this sentence:
Now back to who/whom: who is the subject form of the pronoun, and whom is the object form. So who can only replace subject nouns, and whom can only replace object nouns. (And if you're paying attention, you will now realize that he/she/it are subject pronouns while him/her/them are object pronouns. [Same goes for subjects I/we vs. objects me/us].)
(Isn't grammar completely interesting and exciting!?! ... No?... Oh well...)
In conclusion, I do think it's excusable to say who when whom is correct, mainly for the reason that whom is becoming archaic; many U.S. dialects don't even include whom in the lexicon. However, when people say whom when they should say who, it just looks like they are trying to look sophisticated and intellectual, but instead they are doing themselves a disservice by giving away the fact that they don't understand the difference.
So now that you are properly educated, my dear Bunsnip readers, I expect you all to go forth and mis-whom no more.