I'm no English major, but one thing I'm certain of is objects of a prepositional phrase ALWAYS take the objective form. I'd consider that a prepositional phrase.
Consider these two, and choose: "I got 50 cents on
he". "I got 50 cents on
him". Of course
him, the ojective form, is correct.
[You should not ask] for
whom the bells tolls; it tolls for thee (paraphrased from John Donne). I think that aptly describes the situation here
I'm no English major either (actually I only took one and a half years of high school before dropping out and working full time), but my understanding is that prepositional phrases do not contain verbs. Here is what a quick google search on prepositional phrases turned up:
http://donnayoung.org/english/grammar/prepositions.htm
"Notice that the prepositional phrase contains no verbs. Generally, they contain an adjective, a noun or pronoun and they can also contain a gerund."
Since I had no idea what a gerund is I had to look that up too:
http://www.englishclub.com/grammar/verbs-gerunds_2.htm
"If we want to use a verb after a preposition, it must be a gerund. It is impossible to use an infinitive after a preposition. So for example, we say:
I will call you after arriving at the office.
Please have a drink before leaving.
I am looking forward to meeting you.
Do you object to working late?
Tara always dreams about going on holiday.
Notice that you could replace all the above gerunds with "real" nouns:
I will call you after my arrival at the office.
Please have a drink before your departure.
I am looking forward to our lunch.
Do you object to this job?
Tara always dreams about holidays."
So since "who that guy was" contains a noun and a verb, I think that makes it an independant clause, therefore "that guy/who" would be a subject, not an object? Thus who instead of whom. Perhaps an English major (or someone's wife who has a master's degree in education, Jon73 I'm looking at you), can jump in here and settle this?
Can I just add that in all the months I was lurking here learning how to make beer I never would have guessed my first bunch of posts would be a who vs whom debate. Sorry for making you all suffer through this. This is probably a strange place to do it but thanks to everyone who takes the time to answer everyone's questions, no matter how silly or repetitive. Even though I never actually asked any questions I was there quietly absorbing TONS of useful information. There is beer all over the world that tastes better because of you.