I can never get it straight! When do you use ‘whom’ as opposed to ‘who’? And I need a consistent rule here, not just examples, please.
At the risk of being lambasted for sending you to Google, I found that a search on “+who +whom” turned up several very promising-looking links. In a nutshell, “who” is the subject form (analogous to “he”) and “whom” is the object form (analogous to “him”). But try the Google search I suggested and see if those links help.
Thanks, Scarlett, but I’ve googled and googled and the links never were satisfactorily clear. 
But the subject/object thing I can understand.
Rephrase the question into a sentence using “he” or “him.” Correspondingly, use “who” or “whom.”
“Whom are you speaking about?” = “You are speaking about him.” = correct.
“Who are you speaking about?” = “You are speaking about he.” = wrong.
It’s this simple; “whom” is used when you want to sound like a pompous ass.
Seriously, the only time I ever hear or see this word is when someone is “correcting” someone else’s grammar. Why does it still exist?
First, a note. “Whom” is becoming less and less necessary. You can probably get away with “who” in just about every situation. Now, with that out of the way, here’s the classical answer.
Pronouns can be used in several ways, but the four main ones are subject, predicate nominative, object of a verb, and object of a preposition. For the first two, use the nominative case (who). For the second two, use the objective case (whom).
When these pronouns are used as interrogatives, like Gary T did it, it’s easy. When they’re relative pronouns, however, it’s slightly more difficult. For determining the case, do not look at how the phrase is used in the sentence, but how the pronoun is used in the phrase.
“Who would you like me to kill?”
or
Whom would you like me to kill?
11 years of American public education and you think they could have drummed grammar into me, at least!
Whom! Because you’d say, “You would like me to kill him.”
Gjorp, you use “whom” when you want to be grammatically correct. If using the language properly makes you a pompous ass, then I guess us be in all sorts of trouble.
Just remember, if someone asks: “Who?”
You say “He”
If someone asks: “Whom?”
You say “Him”
Okay, here’s the modern usage that avoids making you sound like a pompous ass but still preserves a little “whom.” Modern usage seems to be, use “who” in all contexts except immediately after a preposition. So you would say, in Standard Informal, “Who are you talking to?” but “the lunatic to whom I was referring,” depending on whether you decided to put the preposition at the end or the beginning of the clause.
Note that this is not, not, not the formal rule. But it seems to be the one that gets exercised the most in standard writing and speech these days.
That’s probably a pretty good idea, matt_mcl, but you should warn that when who/whom is used as a relative pronoun, it’s possible for it to follow a preposition directly and still be in the nominative:
I have no problem with who you are.
I think that most people will recognize the difference between that sentence and this one:
I have nobody with whom to share my problems.
However, I still think one should be careful. In my opinion, the best way to look like an idiot is to use whom when who is correct. 
Thanks for the clarification, Achernar. Hypercorrection, as you mention, sounds absolutely boneheaded. I keep hearing clauses beginning with “whom are,” which make me want to do something rash, like eat a Volkswagen.
Thank you!
Thank you alll; I think I’ve got it now. 
Oh, and matt, you don’t ever eat a Volkswagen; you need to say them for target practice (catch them all)!