I asked my friend Michael LaGory to respond to David's post; rather than embed his response in the comments, I decided to make it a new post.
Grammar haters had better not hold their breath waiting for the day when people stop using “whom” altogether. It is only slightly more imminent than the day people start using it correctly.
I am not impressed by the argument that “whom” does not make a distinction necessary for understanding. The same is true for much more basic rules, such as subject-verb agreement. Few grammatical errors significantly impede understanding, however much they may erode respect.
The “who”-“whom” error is probably most likely in questions (“Who do you love?” “Who can I turn to when nobody needs me?”) and least likely when the pronoun is the object of a preposition. Constructions like “one of whom” are still widely used. I almost never see “one of who.” The tendency to postpone prepositions increases the likelihood of error. Many people whose ears would twitch at “the woman with who I fell in love” would admit “the woman who I fell in love with” without a second thought, their Inner Grammatical Watchdog unstirred.
The Inner Grammatical Watchdog, although widely domesticated, is not extinct. Although the wolf has turned poodle even in many professional writers, in editors like my dear friend Pat, the dominant primordial beast remains alive and snarling. Set one careless foot in her domain, and she’ll be on your case.