Quote Originally Posted by ReDSanchous
Thanks for the explanation!
Well, in the phrase 'To whom it may concern" whom is used because there's a preposition. If I remember correctly, 'who' cannot be used if there's a prepostion in a sentence. For instance, you cannot say: I saw the guy to who you spoke yesterday. You should say: I saw the guy who you spoke to yesterday OR I saw the guy to whom you spoke yesterday.
Perhaps, I'm wrong because it was a very long time ago that I last saw the rule explaining the difference between 'who' and 'whom'.
In "Call whomever you want" whomever is accusative. In "To whom it may concern" whom is dative. Whenever its nominative, you use who. But whenever its not nominative, use whom. So its also "About whom did you speak?" or rather, "Whom did you speak about."

And it should be "I saw the guy whom you spoke to" since whom is not nominative in this sentence.

You are right. It sounds funny to use who after a preposition. In most cases you can use who for whom. After a preposition is one of the few cases where it does sound strange. "I saw the guy who you spoke to" actually sounds more normal than "I saw the guy whom you spoke to" or worse, "I saw the guy to whom you spoke".

But even the sentence, "You spoke to who?" doesnt really sound all that strange.