Stan Carey (You do read his posts, don't you? You should) has an excellent post on the whom quandary, and at Johnson R.L.G. has a follow-up worth reading. As usual, the comments on each article are instructive, though not necessarily in the way the commenters intend. I have trod this hard-packed ground myself at "For whom, the bell tolls" and return only to make some practical remarks.
Historical patterns, usage authorities, questions of register and all else aside, these are things I see as a working editor and teacher.
Colleagues, literate, educated, adult native speakers making a living with words, regularly come up to me to ask, "Should this be who or whom in this sentence?" They're not barbarous. They're not the sort of people who would say, "Her and I went to the movies." They just don't hear it. They don't know whether to use the pronoun as subject or object unless they pause to parse the sentence. And that's in ordinary, journalistic sentences, not rococo Nabokovian constructions.
Even when they do try to work it out, they commonly get it wrong. One of the most common errors it falls to my lot to fix is whom as the subject of a clause that is the object of a verb or preposition. This is why most of the time I advise people, colleagues and students alike, to stick to who, even though whom displays a stubborn survival in written English.*
Very much the same situation obtains with lie/lay, a distinction that my undergraduates look at as if I were attempting to teach them Hittite. There too I see a fading survival in written English alone, and regularly wrong there.
I am perforce a practical man. At the paragraph factory, I do triage on dozens of texts every day. At Loyola, I have fourteen weeks or so to demonstrate and explain micro-editing and macro-editing to undergraduates who have had little reliable instruction in grammar and usage, and evidently even less in analytic thinking. I have to identify what is reasonable to accomplish and worth attempting. Moribund who/whom and lie/lay do not rank very high.