I rarely use 'whom' for the sole reason that it is uncommon and creates a formal distance between speakers. Unfortunately, many people do not understand the history of language and grammar. There are 2 types of grammars: prescriptive and descriptive. Prescriptive grammar defines how a language is "supposed" to be used. Descriptive grammar is how grammar is actually used. Let me give you a very, very brief history of the English language: Prior to the year 1066 (the year of the Battle of Hastings), English had 4 grammatical cases, one of which was the accusative. (Whom is an 'accusative' case. Meaning it is the object of a verb or preposition.) But as English developed due to contact with French, and as is the trend for all Indo-European languages, English lost cases in favor of using prepositions. We still have vestiges of the accusative case (him, her, me), but even some these are falling out of favor in many dialects of English. There is not one English language. There are multiple Englishes. This is a basic fact of English Linguistics. And there was no such thing as a standard English grammar before the 16th century. 16th century grammarians wanted to make English seem more regal, so they arbitrarily took rules from Latin grammar and tacked them on to English even though no native speakers actually talked like this. It is an unfortunate fact that the "correct" (prescriptive) way to speak English always happens to be the way people with economic and political power speak it (*cough* white people *cough*). However, it can easily be proven with linguistic grammar (generative phrase-structure modeling) that many communities of English speakers do not use words like "whom," and yet somehow they all still completely understand each other. How can you tell an entire community of speakers that they are using their own language incorrectly? People insist that those of us who routinely drop words like "whom" from our vocabularies are uneducated. This simply isn't true.

I broadly agree with Brian. It is more common, and not incorrect, to use "who" instead of "whom" in everyday English, although I would make an exception for the case where the pronoun directly follows a preposition (e.g. "to whom" rather than "to who"). I don't think usage is very different between American and UK English. This question comes up all the time here. Here is a more extensive answer with my point of view on the matter: http://classic.italki.com/question/357619

*I have removed my comment because I had no intention of starting a fight or insulting anyone. I am sorry to see certain aspects of the language disappear, but I guess I am alone in that thought.

Personally, as a native English speaker, I have never used the word "whom" except to look up the difference between who and whom. I have heard my English teachers use it occasionally, but certainly not in all of the situations where it theoretically "should have been" used. In extremely formal writing, as in college/job applications, go ahead and use whom if you want. It will make you sound "smart," but you should be wary of using it in conversation, as it can also make you sound like a snob.

If you know when to use "whom", as the interrogative object pronoun, you'll never be wrong to use it. Yes, many English speakers use "who" exclusively, but this hasn't become so common as to be generally accepted, technically correct grammatical usage.