Originally Posted by The Backward OX

I would have responded off the top of my head that "whom" is simply more formal but thought it safer to check first. This is what I found:

(the second paragraph gets more to the nitty-gritty)

—Usage note The typical usage guide statement about the choice between who and whom says that the choice must be determined by the grammar of the clause within which this pronoun occurs. Who is the appropriate form for the subject of a sentence or clause: Who are you? The voters who elected him have not been disappointed. Whom is the objective form: Whom did you ask? To whom are we obliged for this assistance? This method of selecting the appropriate form is generally characteristic of formal writing and is usually followed in edited prose.

In most speech and writing, however, since who or whom often occurs at the beginning of the sentence or clause, there is a strong tendency to choose who no matter what its function. Even in edited prose, who occurs at least ten times as often as whom, regardless of grammatical function. Only when it directly follows a preposition is whom more likely to occur than who: Mr. Erickson is the man to whom you should address your request.
In natural informal speech, whom is quite rare. Who were you speaking to? is far more likely to occur than the “correct” To whom were you speaking? or Whom were you speaking to? However, the notion that whom is somehow more “correct” or elegant than who leads some speakers to hypercorrect uses of whom: Whom are you? The person whom is in charge has left the office. See also than.

Originally Posted by Olly Buckle

Damned if you are doing me Ox! (should there be a comma in that, no, I say it without hesitation.)